Story: Per-sprint capture workflow: individual files and compass integration

The single capture.org file per sprint has become unmanageable: annotations accumulate, implemented items are forgotten, and the file becomes noise rather than signal.

Replace it with product-level captures in the product backlog, where each capture is a separate file scaffolded by compass. New captures land in inbox/ (untriaged). At triage they move to next/ (imminent candidate), deferred/ (long-horizon idea), or discarded/ (explicitly rejected). The LLM calls compass capture --note "..." on the user's behalf; captures can then be promoted into stories or tasks. This makes triage cheap: promote the valuable, file the deferred, discard the stale.

• compass capture --note "..." scaffolds a new capture in doc/agile/product_backlog/inbox/ and prints the path.
• compass capture file <slug> next|deferred|discarded moves a capture into the appropriate backlog bucket.
• compass capture promote <slug> prints instructions to promote a capture to a story or task.
• Four buckets exist under doc/agile/product_backlog/: inbox/, next/, deferred/, and discarded/, each with an index doc.
• The product backlog uses next/ and deferred/ (renamed from near/ and far/).
• The Product backlog knowledge doc documents all four buckets and the capture lifecycle.
• The Document types doc describes the capture type with the four-bucket structure.
• The commit recipe documents [inbox], [next], [deferred], [discarded] as component brackets for backlog commits.
• The Start work on a new story runbook links to the correct compass recipe and the product backlog doc.
• An org-roam capture template in ores.lisp writes new captures to the inbox of the active environment.
• A recipe documents how to write a freeform paragraph and have the LLM decompose it into well-structured captures in inbox/.
• All near / far bucket references across the doc graph have been updated to next / deferred.
• Every entry in sprint 18/capture.org has been triaged into the appropriate product backlog bucket or discarded.

• Migrating existing capture.org files to the new per-file layout (old files are left as-is; the new layout is forward-only).
• Automatic background filing of captures without explicit user instruction.