Phase 1: mechanical echo→chorus rename with back-compat shims

Identity rename, no behavior change (CHORUS-PLAN.md Phase 1):
- Plugin echo-memory → chorus-memory: manifest (v2.0.0-alpha.1), skill dir,
  16 scripts (chorus.py, chorus_config.py, …), EchoError → ChorusError,
  /chorus-* commands, hook paths, docs, scaffold seeds, eval harness,
  build.py. Docs endpoint → chorusapi.mpm.to.
- Env ECHO_* → CHORUS_*; config → ~/.claude/chorus-memory/config.json;
  state dir → ~/.chorus-memory/; marker → _agent/chorus-vault.md.

Back-compat shims (one major version):
- chorus_config aliases legacy ECHO_* env at import; reads a legacy
  echo-memory config.json when no CHORUS config exists (writes never
  land there); doctor/config report the legacy source.
- State dir honors an existing ~/.echo-memory when the new dir is absent
  (offline queue not stranded).
- Marker dual-probe in load/doctor/bootstrap/lint/sweep/migrate: a
  pre-fork _agent/echo-vault.md counts as bootstrapped; bootstrap repair
  won't write a second marker; routing gains agent-marker-legacy (GET).

Verified: 25/25 unit tests, scaffold + routing-sync checks, 4 mock e2e
suites, run_eval metrics unchanged, +6 shim smoke tests green.
Rebuilt chorus-memory.plugin (79 entries).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

chorus-memory.plugin.src/commands/chorus-sweep.md

@@ -0,0 +1,16 @@
+---
+description: Bring the CHORUS vault up to the current feature spec (entity index + cross-links)
+allowed-tools: Bash(python3 *sweep.py*), Bash(python *sweep.py*), Bash(ls /sessions/*), Bash(dirname *)
+---
+
+Use the **chorus-memory** skill to bring the vault up to the current spec. Run the sweep **dry-run first**, show the operator the plan, and only `--apply` on their go-ahead:
+
+```bash
+SDIR="${CLAUDE_PLUGIN_ROOT}/skills/chorus-memory/scripts"
+[ -d "$SDIR" ] || SDIR=$(dirname "$(ls /sessions/*/mnt/.remote-plugins/*/skills/chorus-memory/scripts/sweep.py 2>/dev/null | head -1)")   # CoWork sandbox fallback
+python3 "$SDIR/sweep.py"            # plan (read-only)
+python3 "$SDIR/sweep.py" --apply    # write
+# Windows: use `python` or `py -3` if `python3` is not on PATH.
+```
+
+The sweep (1) creates `_agent/index/`, (2) rebuilds the entity index from existing notes, (3) symmetrizes `## Related` cross-links (adds only the missing reciprocal direction — never invents new links), and (4) stamps the marker `schema_version`. It is idempotent and read-only without `--apply`. If `schema_version` is behind, run `migrate.py` first. After applying, run `/chorus-health` to confirm invariants.