fix(context): honor retrieval.backend config instead of hardcoding embeddings

[dripsmvcp]

Summary

• context._retrieve now reads retrieval.backend from config.yaml and dispatches accordingly, instead of always running embeddings first regardless of config (bug: _retrieve ignores retrieval.backends config and hardcodes embeddings-primary #92).
• Adds embedding as a recognized backend; vouch init writes retrieval.backend: auto; README/ROADMAP/CHANGELOG updated to match real behavior.

Root Cause

src/vouch/context.py:32 called index_db.search_semantic (embedding) unconditionally before FTS5/substring, ignoring config entirely. Meanwhile vouch init wrote retrieval.backends: ["fts5", "substring"] — a list that didn't even contain "embedding" and was never read. So the config knob was decorative and the README/ROADMAP claim ("Default stays FTS5; embeddings opt-in") was wrong.

Fix

• New _configured_backend(store) reads retrieval.backend (auto|embedding|fts5|substring, default auto), with back-compat for a legacy retrieval.backends list (first recognized entry wins).
• _retrieve dispatches: auto = embedding → FTS5 → substring (degrades to FTS5 when embeddings extras aren't installed); embedding/fts5/substring pin a single path. Mirrors kb_search backend semantics; preserves the existing sqlite3.Error fallback.
• _starter_config now writes retrieval.backend: auto.
• Docs: ROADMAP marks embeddings shipped + describes the knob; README Retrieval row rewritten and the stale "vector embeddings (BM25/FTS5 only)" line removed.

Test Plan

• Regression test added: tests/test_retrieval_backend.py (5 tests, env-independent — monkeypatches search_semantic so no numpy/sentence-transformers needed)
• test_backend_fts5_skips_embedding and test_backend_substring_only fail on the parent commit, pass with the fix
• ruff + mypy clean; full suite green except one pre-existing failure (test_crystallize_cli_partial_failures_shows_warning) that also fails on clean upstream/test (environment-sensitive _whoami())

$ python -m pytest tests/test_retrieval_backend.py -q
.....                                                  [100%]
$ python -m ruff check src tests
All checks passed!
$ python -m mypy src
Success: no issues found in 31 source files

Closes #92

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f9f92d2e-8591-4b1b-821f-0d3102832cbd

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}