feat(agent): isolate agent cwd in .ideai/run/<id> to kill convention-file collisions

ARCHITECTURE §14.1: an agent's PTY cwd is now its own
`<project_root>/.ideai/run/<agent-id>/` directory, never the project root, so
N agents of the same profile no longer collide on a single conventional file
(CLAUDE.md/AGENTS.md/...).

- profile: cwd_template is now "{agentRunDir}" (built-in catalogue + docs).
- runtime: resolve_cwd substitutes {agentRunDir} (legacy {projectRoot} kept).
- LaunchAgent: computes + creates the run dir via FileSystem::create_dir_all,
  passes it as the cwd base to the pure prepare_invocation. Contract chosen:
  pass run_dir as the `cwd` argument (no PreparedContext change) — keeps
  prepare_invocation pure, I/O stays in the use case.
- convention file is generated by IdeA inside the run dir via a pure
  compose_convention_file(project_root, agent_md): absolute project-root header
  + agent persona (extensible for skills, §14.2).
- .gitignore: ignore .ideai/run/.
- run-dir cleanup left as a TODO (FileSystem port exposes no delete).

Tests: anti-collision (2 agents -> 2 distinct cwd, 2 distinct convention files,
none at root), run-dir creation order, composed convention file; pure unit
tests for agent_run_dir + compose_convention_file; runtime {agentRunDir}
substitution. cargo test --workspace + clippy -D warnings green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -25,6 +25,11 @@ frontend/coverage/
 # Personal, machine-local overrides (shared settings.json, if any, stays tracked).
 .claude/settings.local.json
 
+# ─── IdeA project data ──────────────────────────────────────────────────────
+# Ephemeral per-agent run directories (isolated PTY cwd + generated convention
+# files), created at activation — not versioned (ARCHITECTURE §9.1 / §14.1).
+.ideai/run/
+
 # ─── Editors / OS ───────────────────────────────────────────────────────────
 .idea/
 .vscode/