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>

CHANGELOG.md

@@ -1,5 +1,38 @@
 # Changelog
 
+## 2.0.0-alpha.1 — CHORUS fork, Phase 1 (mechanical rename)
+
+CHORUS is a group-based fork of ECHO (`jason/echo`, v1.5.1, schema 4 — preserved at the
+`echo-fork-point` tag). Phase 1 is the identity rename with no behavior change; see
+`CHORUS-PLAN.md` for the full conversion plan.
+
+### Changed
+- Plugin renamed `echo-memory` → `chorus-memory`; skill dir, all 16 `echo*.py` modules
+  (`chorus.py`, `chorus_config.py`, …), `EchoError` → `ChorusError`, the eight slash
+  commands (`/echo-*` → `/chorus-*`), hook script paths, and all docs/scaffold seeds.
+- Env vars renamed `ECHO_*` → `CHORUS_*`; config moves to
+  `~/.claude/chorus-memory/config.json`; local state dir to `~/.chorus-memory/`;
+  bootstrap marker to `_agent/chorus-vault.md`; manifest version → 2.0.0-alpha.1.
+- Docs' endpoint references now `chorusapi.mpm.to`.
+
+### Back-compat (one major version)
+- Legacy `ECHO_*` env vars are adopted as `CHORUS_*` at import when the new name is unset.
+- A legacy `~/.claude/echo-memory/config.json` is read (never written) when no CHORUS
+  config exists; `config`/`doctor` report the legacy source.
+- A legacy `~/.echo-memory/` state dir is honored when `~/.chorus-memory/` doesn't exist
+  yet, so queued offline writes aren't stranded.
+- The bootstrap probe (`load`, `doctor`, `bootstrap`, `vault_lint`, `sweep`, `migrate`)
+  falls back to a pre-fork `_agent/echo-vault.md` marker; bootstrap's repair pass leaves
+  a legacy marker in place (the schema migration will rename it) rather than writing a
+  second marker. `routing.json` gains a read-only `agent-marker-legacy` route.
+
+### Verified
+- 25/25 client unit tests, scaffold checks, routing-doc sync, and all four mock
+  end-to-end suites green; `run_eval.py` metrics unchanged (0 dupes / 0 false blocks /
+  0 silent failures / 0 lost offline writes). Six new shim smoke tests (legacy env,
+  legacy config path, chorus-over-legacy precedence, legacy-marker load/doctor, fresh
+  vault) pass.
+
 ## 1.5.1
 
 ### Fixed — duplicate gate over-firing on vault-common tokens and cross-kind names