Task: Stamp the working environment on documents compass touches

Add #+environment: stamping to compass so every task and story records which checkout worked on it. The value comes from ORES_CHECKOUT_LABEL in the repo's .env file. Compass writes the field on task start (task

• story) and task done (task only). New task and story templates gain a

blank #+environment: placeholder so the field is always present after scaffolding.

• Tasks record the environment that worked on them (stamped by compass task start/done from the .env label).
• Stories record the environment that started them (stamped when the story moves to STARTED).
• Existing documents are unaffected until next touched; no backfill.
• Note: this stamp is the timeline's only attribution mechanism (journals are not a timeline input), so this task gates the snapshot skill.

(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.)

• Added _set_frontmatter_field(path, field, value) helper to compass.py (after _set_frontmatter_branch): updates an existing #+field: line or inserts after #+updated: (or #+created: as fallback).
• Wired env stamp into _cmd_task_start: reads ORES_CHECKOUT_LABEL from .env and calls _set_frontmatter_field on both the task and the story (when present).
• Wired env stamp into _cmd_task_done: stamps the task only.
• Added blank #+environment: field to doc_task_org.org / doc_task.org.mustache and doc_story_org.org / doc_story.org.mustache (after #+updated:).