v0.4.2 — auto-merge on Done, board-viewer integration, StreamReader fix

Builtin auto-merge on Done, board-viewer launcher integration, and a
StreamReader limit fix that unblocks long agent turns. Drop-in over
0.4.1; auto-merge is ON by default but safe-by-default (dirty host or
missing branch skips silently).

Added

• agent.auto_merge_on_done (default true) — when a ticket reaches
  Done, fold the symphony/<ID> branch into the host repo's
  development branch as one selective-apply commit. Paths in
  agent.auto_merge_exclude_paths (default
  kanban/llm-wiki/prompt/docs) are stripped first so the workspace
  symlinks that hooks.after_create installs never reach the host
  repo. agent.auto_merge_target_branch defaults to "" = the host
  repo's currently-checked-out branch.
• tui-open.sh now auto-starts tools/board-viewer/server.py in the
  background at http://127.0.0.1:8765/ when the workflow ships one.
  Skipped silently if the port is held or the file is absent.
• src/symphony/auto_merge.py — new module owning the selective-apply
  flow with five outcome events: auto_merge_completed,
  auto_merge_skipped_dirty, auto_merge_skipped_missing_branch,
  auto_merge_nothing_to_apply, auto_merge_failed.
• Learn-stage wiki entries now open with a beginner explainer block
  (## 감 잡기 in Korean, ## Getting the Feel in English) ahead of
  the existing Summary / Invariants / Files / Decision-log technical
  reference. Same file, two layered audiences — PMs and 기획자 land on
  the beginner block, engineers scroll past it. The prompt enforces the
  tutor shape: 3-5 step core flow, exactly five plain-language terms,
  one realistic scenario, one-sentence takeaway, and a "ready to go
  deeper" pointer. Branch is {% if language == 'ko' %} keyed on the
  same {{ language }} env that drives the chrome/doc directive, so
  switching the TUI language (or SYMPHONY_LANG) flips both halves of
  the wiki entry in lockstep. Both docs/symphony-prompts/file/stages/learn.md
  and docs/symphony-prompts/linear/stages/learn.md updated.

Fixed

• claude_code, pi, gemini backends now spawn their subprocess with
  limit=10 MiB on the asyncio StreamReader, matching codex. The
  asyncio default of 64 KiB was raising LimitOverrunError on
  stream-json events whose result/tool-use payload exceeded that on a
  single line, dropping the rest of the turn into stalled_session
  recovery. Caught on a live IB-002 turn before the fix landed.
• _on_worker_exit(reason="normal") at Done now fires
  auto_merge_on_done, after_done user hook, and workspace.remove
  inline. Previously those only ran on the reconcile-driven termination
  path; a worker that finished cleanly at Done was popped from
  _running before the next reconcile cycle, so the entire
  terminal-state post-processing was silently skipped — the host repo
  never saw the auto-merge commit and the workspace lingered.
  _reconcile_running remains the safety net for stale workers.

Tests

• tests/test_auto_merge.py — five scenarios: happy path,
  dirty host, missing branch, implicit current-branch target,
  all-paths-excluded. Full suite: 353 passed, 6 skipped.