refactor(claude-md): split shared base into module fragments, inject name at runtime

Move every agent-specific instruction out of the shared container/CLAUDE.md
so the base is genuinely universal. Persona/identity now comes from the
system-prompt addendum (buildSystemPromptAddendum now takes assistantName
and prepends "# You are {name}"). Per-module instructions live alongside
each MCP tool source:

  container/agent-runner/src/mcp-tools/core.instructions.md
  container/agent-runner/src/mcp-tools/scheduling.instructions.md
  container/agent-runner/src/mcp-tools/self-mod.instructions.md

composeGroupClaudeMd() scans that directory and emits `module-<name>.md`
fragments as symlinks to /app/src/mcp-tools/<name>.instructions.md (valid
via the existing RO source mount). Skill fragments renamed to
`skill-<name>.md` for naming consistency with `module-*` and `mcp-*`.

Mount tightening so composer-managed files can't be clobbered by agent
writes: nested RO mounts for /workspace/agent/CLAUDE.md and
/workspace/agent/.claude-fragments/. CLAUDE.local.md (per-group memory)
stays RW as the only writable CLAUDE.md-family file.

.gitignore: ignore CLAUDE.local.md, .claude-shared.md, .claude-fragments/
everywhere, and simplify groups/ rules to ignore the whole tree (per-
installation state, not tracked).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -11,14 +11,14 @@ store/
 data/
 logs/
 
-# Groups - only track base structure and specific CLAUDE.md files
+# Groups - per-installation state, not tracked
 groups/*
-!groups/main/
-!groups/global/
-groups/main/*
-groups/global/*
-!groups/main/CLAUDE.md
-!groups/global/CLAUDE.md
+
+# Composer-managed CLAUDE.md artifacts (regenerated every spawn) and
+# per-group memory (CLAUDE.local.md) must never be committed.
+**/CLAUDE.local.md
+**/.claude-shared.md
+**/.claude-fragments/
 
 # Secrets
 *.keys.json