feat(session-flow): send /session-flow:retro findings the user does not apply to the findings buffer #12152

Parent #7575 records D50 item 10, approved by the owner on 2026-10-13 (poteto-review decision row 50): "Retro offered at chain end, not forced; unapplied lessons go to the buffer". The offer half is on main: /session-flow:workflow lists "Retrospective (optional)" at the end of its chain (plugins/session-flow/skills/workflow/context/steps.md:187-190). The buffer half is not: /session-flow:retro never mentions the findings buffer. The buffer is an append-only pool nobody fixes on sight, read through a grouped view (D50, D62); /corrections:codify note "<observation>" appends one row to it (plugins/corrections/skills/codify/SKILL.md:32). The reason for the route: a lesson the user declines today is lost, so the next session cannot see it recur, and /corrections:codify group counts recurrences against the codify threshold.

Done when

• plugins/session-flow/skills/retro/context/session.md, under "Phase 4: Act (with user approval)", says that each finding the user declines or leaves unselected is appended to the findings buffer through /corrections:codify note, one call per finding; applied findings and "Queue for follow-up" items are not appended.
• The same text says what happens when the corrections plugin is not installed: the unapplied findings are listed in the retro report under the heading "Unapplied findings", and no file is written.
• The text says each note's observation is one sentence stating the lesson, with no quoted transcript text and no person named, as /corrections:codify's note action requires.
• The diff edits neither plugins/session-flow/skills/retro/SKILL.md nor plugins/session-flow/skills/retro/evals/evals.json.
• A .changes/session-flow/*.md fragment with bump: patch describes the change.
• The PR's CI is green.

Out of scope

• Changing the chain-end offer: it is already optional.
• Changing the findings buffer or /corrections:codify.

Parent: #7575