fix: capture OpenCode sessions that predate the session-tracker (#402)

[dhague]

Problem

enchiridion save-session refused to capture an OpenCode session that was started before the session-tracker plugin was installed, exiting non-zero with:

No state recorded for session <id> under .opencode/wiki-knowledge/sessions, per the session-tracker plugin.

The transcript then had to be reconstructed by hand. Fixes #402.

Root cause

captureOpenCodeSession required a recorded <id>.json state entry (via findOpenCodeSessionID, which threw). But opencode export <id> returns the transcript whether or not the SessionStart hook ever fired — so tracker state was never a real prerequisite for the capture, only tie-break evidence.

Fix (issue option 2)

Split findOpenCodeSessionID into two intents:

• openCodeSessionIDFromEnv — reads $OPENCODE_SESSION_ID, the only hard prerequisite for a capture. captureOpenCodeSession now uses this and always shells out to opencode export, tracked or not.
• isOpenCodeSessionTracked — a never-throwing boolean, consulted only as tie-break evidence in captureSession when both host session-id variables are set. Behaviour there is unchanged: an untracked OpenCode id still yields to Claude Code (an untracked id can be a leaked variable from an unrelated project).

Tests

• New: captureOpenCodeSession captures a session that predates the tracker (no .opencode/ state at all) via the injected export seam.
• New: unit coverage for openCodeSessionIDFromEnv (throws unset / returns id) and isOpenCodeSessionTracked (all four false branches + the true branch).
• Updated: the only-$OPENCODE_SESSION_ID-set dispatch test now asserts the export path succeeds without tracker state, rather than failing.

npm run typecheck, npm run lint clean. Full suite passes except pre-existing Windows-only failures (subprocess .bin/tsx spawn; tests assuming no .claude/ ancestor) — CI runs on Linux and is unaffected.

Bundle artifacts are intentionally not rebuilt here — the shipped bundle is refreshed at release time (scripts/release.sh); the CI freshness guard only runs on version-bump PRs.