fix: honest session_state source label — "compaction" only after real compaction

[donwellsav]

Problem

hooks/auto-injection.mjs hardcodes the wrapper label:

return `<session_state source="compaction">\n\n${parts.join("\n\n")}\n\n</session_state>`;

but the pi adapter injects that block on every before_agent_start — not
just after a real compaction (the code comment says so itself: "Pi-3 + Pi-4:
Always build active_memory (not just post-compact)"). The file header of
auto-injection.mjs still documents the original invariant ("Only fires on
source === 'compact'"), which the pi adapter silently broke.

Observed impact (2026-07-24): a pi session with context-mode active had
zero compaction events in its transcript, yet the standing
source="compaction" marker led two different models to declare their
context destroyed and ask the user to re-provide decisions that were fully
visible in context — a false "context loss" cascade.

Fix

The label now tells the truth, on every path:

• buildAutoInjection(events, source) — source is a required
  parameter (fail-safe: no default, so no present or future caller can
  silently emit the wrong label).
• Fidelity line only on genuine compaction — "Context was compacted; full history persists in the session transcript." tells the agent what
  happened and where its history lives (the two facts missing in the
  incident). Routine per-turn injection carries no extra line, so the
  ~500-token content budget and per-turn token cost are unaffected.
• pi adapter tracks session_compact via _pendingCompactLabel: the
  first before_agent_start after a real compaction is labeled
  "compaction"; all routine turns are labeled "active_memory". The flag
  is read-and-cleared at the top of the handler (can never strand true and
  mislabel a later unrelated turn) and reset in session_shutdown (can never
  leak across sessions in the same process).
• Genuine compaction paths pass "compaction" explicitly:
  hooks/sessionstart.mjs (source === "compact") and the opencode plugin
  (experimental.session.compacting) — the latter would otherwise have been
  relabeled active_memory, inverting the bug on that platform.

Tests

New tests/auto-injection-label.test.ts (5 cases): both labels, fidelity
line presence/absence, empty-events path, required-parameter loudness, and an
honest budget assertion (content ≤ 500 tokens + unbudgeted wrapper/fidelity
overhead; P2-overflow fixtures excluded by design).

• New tests: 5/5 pass
• tests/pi-extension.test.ts + tests/opencode-plugin.test.ts: 125/125 pass
• tsc --noEmit: clean
• Full suite: 4721 passed / 1 failed — the failure
  (tests/core/server.test.ts › CONTEXT_MODE_PROJECT_DIR env cascade) is
  pre-existing and environment-related: it fails identically on a clean
  checkout of main in the same environment and does not touch this code
  path.

Notes for review

• build/pi-extension.js in the published tarball contains a stale duplicate
  of the per-turn injection call site. Nothing in the package references it
  (package.json → pi.extensions points at build/adapters/pi/extension.js);
  it may be worth deleting in a separate cleanup.
• Behavior change is limited to label text + one added sentence after real
  compactions; no event-selection, priority, or budget logic was modified.