refactor(session): drop the dead mutable SessionSummary

SessionSummary (updatedAt/title/firstPrompt) and SessionPersistence.update()
were dead state: zero production callers of update(), no production reader of
updatedAt/firstPrompt, and ACP's title comes from a tool-call presenter, not
storage. The live Session.header was already typed SessionHeader, so the
summary only ever existed in the persistence layer, written and read by nothing
but its own contract test.

Delete it entirely (no SessionMeta alias — SessionMeta collapses to
SessionHeader everywhere). This removes the JSONL .summary.json sidecar
machinery, the SQLite title/first_prompt/updated_at columns and per-append
updated_at bump, and the update() method from the abstract service and both
backends. SQLite SCHEMA_VERSION goes 1->2 and openDatabase now rejects any
non-current user_version (older or newer) — no migration, unreleased software.

Net -400 lines, and it erases the JSONL-sidecar-vs-SQLite-column durability
divergence that the upcoming write coordinator would otherwise have to model.

Records the decision in docs/rfc/implemented/2026-06-19-drop-mutable-session-summary.md
and migrates the 2026-06-14 session-persistence RFC's facts to current truth.
Adds a standalone AGENTS.md section "Tests document behavior, not golden truth"
(a passing test pins current behavior, not necessarily correct behavior) with
the summary-drop as its worked example, and reinforces the no-migration
pre-release stance.

AGENTS.md

@@ -6,6 +6,16 @@ This is the monorepo for the DeepSeek Harness group. It currently hosts the code
 
 **This applies only while the harness is unreleased — remove this section at the first tagged/published release.** There are no external consumers yet, so optimize for the *correct foundation*, not for a small diff. When the right structure means moving a file across package boundaries, renaming a public symbol, or repackaging a plugin, do it — and update every reference in the same change. Do **not** add backward-compat shims, deprecation aliases, re-export stubs, or "keep it where it is to avoid churn" hedges; those are debts you take on to protect callers you do not have. Churn now is cheap; a wrong foundation set in stone is not. (Once released, this inverts — backward compatibility becomes a real constraint and this section comes out.)
 
+This extends to **on-disk formats, schemas, and stored data**: while unreleased there is no persisted user data to preserve, so a format/schema/contract change needs **no migration path**. Bump the version and reject (don't migrate) anything not at the current version — e.g. the SQLite backend's `SCHEMA_VERSION` bump that drops columns simply rejects any non-current `user_version` on open, with no v1→v2 migration. A migration written now is a shim for data that does not exist.
+
+## Tests document behavior, not golden truth
+
+A passing test pins the behavior the code **currently** has — not necessarily the behavior it **should** have. Existing tests faithfully document existing behavior, but existing behavior is not automatically golden: it can be the residue of a past compromise, a half-built feature, or a limitation that no longer applies. So when a refactor or review makes you ask "can I change this?", a green test is **not** the answer — the question is whether the behavior the test pins is actually correct.
+
+Before you preserve a behavior solely to keep a test green, ask: is this behavior load-bearing (a real consumer depends on it, a contract promises it, a user observes it), or is it an artifact? If it's an artifact, **change the behavior AND its test together, in the same change, and say why in the PR** — do not contort new code to keep an obsolete assertion passing, and do not treat "but the test expects X" as a reason X must stay. Conversely, do not delete a test just because it is inconvenient: the discipline cuts both ways — you must show the *behavior* is dead, not merely that the test is in your way.
+
+The worked example is [Drop the mutable session summary](docs/rfc/implemented/2026-06-19-drop-mutable-session-summary.md): an entire `SessionSummary` type, a `SessionPersistence.update()` method, a JSONL sidecar, and SQLite columns existed and were exercised by their own contract test — yet nothing in production read or wrote any of it. The tests documented the behavior perfectly; the behavior was dead. Deleting the behavior and its tests together removed ~400 lines and erased a durability divergence the next refactor would have had to model. (This is the test-tier echo of "verify the world, not a synthetic stand-in" in § Defensive patterns: a test agrees with whatever it was written to assert; only a real consumer proves the behavior matters.)
+
 ## Architecture
 
 This codebase is based on the **Cordis** framework, built microkernel-style: **everything is a plugin**. All necessary Cordis dependencies are copied into this monorepo as vendored source (under `vendor/`) instead of being depended on via npm.