feat(story,role): add retro ceremony -- closes the self-improvement loop (Phase 8)

The final mechanism the versioned role-config model (Phase 5) was built for:
when a story reaches DONE, StoryOrchestrator spawns a retro-role task that
reflects on the story's full history and proposes draft role_configs
versions for a human to review and activate via the existing (unchanged)
POST /api/roles/{role}/activate.

- AgentChannel gains a 6th method, ProposeRoleConfig(ctx, role.RoleConfig)
  (version, err), following ProposeEpic's precedent (Phase 7c): a structured
  tool call, not summary-parsing. storeChannel.ProposeRoleConfig calls the
  same Store.CreateRoleConfig the human-facing POST /api/roles/{role}/versions
  endpoint already uses (proposed_by: "retro"), landing a new draft row
  without touching whatever's currently active. Wired through both
  transports exactly like ProposeEpic: internal/agentloop/tools.go (native
  loop) and internal/executor/agentmcp.go (MCP).
- StoryOrchestrator.Tick now routes a story at status DONE to a new
  processRetro stage instead of processStory -- a sibling stage, not a
  continuation, since the Builder->Evaluators->Arbitration chain is long
  settled by then. processRetro only *reads* that settled pipeline
  (read-only findEvaluators/findArbitration counterparts to
  ensureEvaluators/ensureArbitration -- it never spawns/mutates
  Builder-pipeline tasks) to locate the Arbitration task the retro task
  depends on, then spawns (idempotently -- checks for an existing
  retro-role dependent first) one retro-role task with instructions
  assembled from the story's spec/acceptance-criteria, full task tree, per-
  task cost/escalation history, active role_configs per role encountered,
  and the story's own event stream (evaluator verdicts, arbitration
  decision).
- event.KindRetroCaptured (attached to the story's ID, matching
  KindEvalVerdict/KindArbitrationDecided's convention) fires once the retro
  task completes (auto-accepted like every other pipeline task), aggregating
  every event.KindRoleConfigProposed the retro task recorded (one per
  propose_role_config call) into {task_id, proposals: [{role, version}],
  summary} -- the summary is the "capturing lessons" half of this ceremony,
  the proposals are the versioned-config half.
- Human activation is completely untouched: drafts land through the
  identical CreateRoleConfig/config_json path Phase 5's endpoints already
  handle, confirmed via existing role-endpoint tests passing unmodified.

go build/vet/test -race -count=1 all pass, full suite (20 packages) -- one
run hit a known, pre-existing, intermittent flake under full-suite load
(unrelated to this phase's files) that did not reproduce on two immediate
reruns, both in isolation and full-suite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V1moSNCJRcP6kykA4tyUSs

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