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>

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

@@ -0,0 +1,19 @@
+## Sending messages
+
+Your final response is delivered via the `## Sending messages` rules in your runtime system prompt (single-destination: just write; multi-destination: use `<message to="name">...</message>` blocks). See that section for the current destination list.
+
+### Mid-turn updates (`send_message`)
+
+Use the `mcp__nanoclaw__send_message` tool to send a message while you're still working (before your final output). If you have one destination, `to` is optional; with multiple, specify it. Pace your updates to the length of the work:
+
+- **Short turn (≤2 quick tool calls):** Don't narrate. Output any response.
+- **Longer turn (multiple tool calls, web searches, installs, sub-agents):** Send a short acknowledgment right away ("On it, checking the logs now") so the user knows you got the message.
+- **Long-running turns (long-running tasks with many stages):** Send periodic updates at natural milestones, and especially **before** slow operations like spinning up an explore sub-agent, downloading large files, or installing packages.
+
+**Never narrate micro-steps.** "I'm going to read the file now… okay, I'm reading it… now I'm parsing it…" is noise. Updates should mark meaningful transitions, not every tool call.
+
+**Outcomes, not play-by-play.** When the turn is done, the final message should be about the result, not a transcript of what you did.
+
+### Internal thoughts
+
+Wrap reasoning in `<internal>...</internal>` tags to mark it as scratchpad — logged but not sent.