Task: Implement ores.shell market data import command

Add a command to ores.shell that reads a market-data file (ORE market.txt=/=fixings.txt format) and an optional --source tag, then sends import_market_data_request over NATS (marketdata.v1.import) exactly as ImportTradeDialog does today — same request/response, same import_service, just a non-interactive entry point.

• ores.shell gains an import command taking --file=/–fixings= paths and an optional --source tag. ✓
• Round-trips successfully against a representative ORE example vintage. ✓ — verified against Academy/TA002_IR_Swap and MarketRisk/HistSimVar (including real fixings import); the originally-named Legacy/Example_56 vintage still fails, but on a separate, pre-existing import_service bug (Full market.txt import hits duplicate observation key) unrelated to this command — the command itself sends the request correctly in all cases.
• A recipe documents the command, following the existing ores.shell command doc convention. ✓ — three recipes, one per tested vintage.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

No marketdata_commands.* exists yet in ores.shell — this is a new command file, modelled closely on projects/ores.shell/src/app/commands/bundles_commands.{hpp,cpp} (its process_publish is the template: parse_args, an authenticated_request + rfl::json round-trip via a local do_auth_request<Response> helper, success/failure output). No existing shell command currently slurps a file into a std::string for a NATS payload (the only file reader, script_commands.cpp's load, streams line-by-line instead), so a small local file-read helper is new code here, not reuse.

1. New header projects/ores.shell/include/ores.shell/app/commands/marketdata_commands.hpp
   • Class marketdata_commands with logger_name, register_commands(cli::Menu&, nats_client&) and process_import(std::ostream&, nats_client&, const std::vector<std::string>&), mirroring bundles_commands.hpp's shape.
2. New source projects/ores.shell/src/app/commands/marketdata_commands.cpp
   • Includes ores.marketdata.api/messaging/import_protocol.hpp, ores.nats/domain/message.hpp, command_args.hpp, command_feedback.hpp, rfl/json.hpp.
   • Local anonymous-namespace helpers: a do_auth_request<Response> copy (same pattern as bundles_commands.cpp — no shared header exports it) and a read_file(const std::string& path) -> std::optional<std::string> that opens an std::ifstream, returns std::nullopt on failure, else slurps via std::ostringstream << file.rdbuf().
   • register_commands: builds a cli::Menu("marketdata") submenu with one command, import: usage marketdata import [--file <path>] [--fixings <path>] [--source <tag>].
   • process_import:
     • parse_args with flags file, fixings, source (all requires_value=true, empty default).
     • Reject if both file and fixings are empty (nothing to import) — usage message.
     • Reject if not session.is_logged_in(), matching bundles_commands.
     • Read each non-empty path via read_file; fail loudly (path in the message) if a read fails.
     • Build import_market_data_request with market_data_content, fixings_content, source (from the flags), send via do_auth_request<import_market_data_response> using req.nats_subject and a generous timeout (mirror publish_timeout's 5 minutes — imports can be large).
     • On failure: fail(out) << result->message.
     • On success: print series_count=/=observation_count=/=fixing_count and result->message.
3. Register the command in projects/ores.shell/src/app/repl.cpp: add #include "ores.shell/app/commands/marketdata_commands.hpp" (alphabetical, after lei_commands.hpp) and marketdata_commands::register_commands(*root, session_); next to the other register_commands calls.
4. Link the new dependency: add ores.marketdata.api.lib to the PRIVATE list in target_link_libraries in projects/ores.shell/src/CMakeLists.txt (sources are globbed, so no other CMake change is needed for the new .cpp).
5. Verify by building ores.shell, logging in, and running marketdata import --file external/ore/examples/Legacy/Example_56/Input/market.txt --source ore.reference against a running environment; confirm series_count=/=observation_count match expectations and that market_observation rows land tagged source = "ore.reference".
6. Document: add a recipe under doc/recipes/shell/marketdata/ via the doc-add-recipe skill, following the existing ores.shell recipe convention (org metadata drawer, ores-shell babel block with the literal REPL session, #+RESULTS:).
7. Once this task is DONE and the story closes, resume the blocked task Import the 2016-02-05 ORE reference vintage using this new command.

Shipped marketdata import [--file <path>] [--fixings <path>] [--source <tag>] (marketdata_commands.{hpp,cpp}), following the plan exactly. Verified end to end against a freshly-provisioned Barclays tenant:

• Academy/TA002_IR_Swap: ✓ Imported 4 series, 52 observation(s), 0 fixing(s) (this vintage's fixings.txt is empty, so 0 is expected).
• MarketRisk/HistSimVar: ✓ Imported 217 series, 7172 observation(s), 904 fixing(s) — real fixings import confirmed working.
• Legacy/Example_56 (2016-02-05, the vintage the blocked sibling task actually needs): fails deterministically on a Postgres unique- constraint violation in market_observations. Confirmed this is a pre-existing =import_service=/series-key-decomposition bug, not a shell-command bug — captured separately at Full market.txt import hits duplicate observation key.

Along the way, found and fixed two real import_service bugs surfaced by this command's first non-interactive, full-file exercise (not caught in review because the mechanism was previously only exercised for a handful of curated quotes):

• change_reason_code was hardcoded to the literal "IMPORT", which doesn't exist in ores_dq_change_reasons_tbl — now dq::domain::change_reason_constants::codes::external_data_import.
• modified_by=/=performed_by were hardcoded to "import", which fails the account/service-role validation trigger — now ctx_.actor()=/=ctx_.service_account(), matching every other service.

Documented via three recipes under doc/recipes/shell/marketdata/ (one per tested vintage, tangled into projects/ores.shell/scripts/library/marketdata/), including an honest "currently broken" recipe for Example_56 linking the tracking capture rather than hiding the failure.

Follow-up: resume Import the 2016-02-05 ORE reference vintage once the duplicate-key bug is fixed.