docs: make capturing lessons learned an explicit done gate

[bkrabach]

Summary

This change makes "capturing lessons learned back into per-repo convention files" an explicit part of the "done" requirements for any work session, closing a gap where important lessons surfaced during development were never saved back into the conventions.

Motivation

Today, agents and developers discover footguns, refine invariants, and reach team agreements about how work should be done — but these lessons often evaporate at session end rather than becoming part of the collective knowledge encoded in AGENTS.md, PRINCIPLES.md, PR templates, and KNOWN_ISSUES.md. This means the next session re-arms the same footguns and re-litigates the same design questions.

What Changed

In docs/PER_REPO_CONVENTIONS.md:

1. New lifecycle gate — Added "Capturing what you learned" to the "Honor what you read" list (the list of obligations that close a work session), making lesson-capture an explicit done criterion alongside PR templates, gate execution, and deferral logging.

2. Dual capture discipline — Requires capturing lessons both inline as the lesson lands and as a session-end sweep before declaring work done, providing belts-and-suspenders protection against lost context. Uses the same offer-and-confirm discipline as deferral to prevent becoming a dumping ground.

3. Awareness-boundary routing — Adds an enforcement rule: lessons may only be written into a repo when that repo is allowed to know what the lesson describes. Anchors to amplifier's REPOSITORY_RULES.md as the canonical authority for repo awareness boundaries, preventing lessons about dependents from leaking into agnostic upstream repos.

4. Consistency updates — Refreshes owner guidance, cross-references, and TL;DR to reflect the new gate and cross-link to the routing rule.

Design Rationale

• Belts and suspenders: Inline capture catches lessons hot; session-end sweep catches ones that fell out of context.
• No checklist for what's worthy: Trust the model's judgment. A footgun, a fixed invariant, or an agreed design decision is worth capturing; passing TODO noise is not.
• Routing defers to existing authority: Rather than re-derive repo awareness rules, anchor to the existing REPOSITORY_RULES.md table (Repo-Type → Awareness-Rule), so lessons respect the same boundaries as all other cross-repo knowledge.

This prevents a category of silent regressions: sessions that pass all gates, tests, and PRs but leave the next session unprotected against the same class of bugs, or re-discovering the same design constraints.