fix(pi): dedupe active_memory injection across turns (#1007)

[Nithin745]

Fixes #1007

Root cause

before_agent_start unconditionally rebuilt _pendingContext from
db.getEvents(sessionId, { minPriority: 3, limit: 50 }) on every turn where
that query returned any rows — even when the returned events were identical
to what had already been injected on a prior turn.

The "context" hook pushes _pendingContext as a transient message that is
never persisted back into context.messages (transformContext only
mutates a local copy of the message array for that one outgoing call), so a
repeated-but-never-cached-in-this-exact-form message lands right where
Anthropic's cache_control breakpoint sits — a full cache-miss rewrite
instead of a cache-read on every such turn. This is the exact reproduction
documented in #1007 (session events vs. Anthropic usage deltas in a live
transcript).

Fix

The resume-snapshot injection right next to this code already solves the
identical problem with a "consumed" flag (db.markResumeConsumed), shown
exactly once. This PR applies the same high-water-mark pattern to
active_memory:

• A new module-level _lastShownActiveMemoryEventId (mirroring the existing
  single-session-per-process module state already in this file, e.g.
  _sessionId / _pendingContext) tracks the max session_events.id
  already shown as active_memory in the current session.
• before_agent_start now only considers events newer than that mark. If
  nothing qualifies, no active_memory block is built and no cache-miss
  message is added for that reason.
• The mark advances to cover every newly-considered event once
  memoryContext is non-empty, and resets on session_start /
  session_shutdown.

All existing behavior is preserved: the minPriority: 3 filter, the
limit: 50 cap, the role category exclusion (#856), the
buildAutoInjection 500-token-budget helper with its inline fallback
formatting, and the resume-snapshot logic (untouched).

Note: the always-on routing anchor (Pi-1) still fires every turn regardless
— that's an intentional, separate, fixed-size injection out of scope for
this issue, which is specifically about active_memory repetition.

Tests

Added an "Issue #1007" describe block in tests/pi-extension.test.ts with
two cases, following the file's existing conventions:

1. Reproduces the bug: seeds one priority-3 non-role session event,
   confirms it's injected on the first before_agent_start turn, then
   confirms a second turn with no new qualifying events does not repeat
   that content.
2. Confirms the fix scales: a new qualifying event landing between two
   turns causes the second turn's injection to contain only the new
   event, not the previously-shown one again.

Both new tests were confirmed to fail against the pre-fix code (verified
by temporarily reverting the src/adapters/pi/extension.ts change and
re-running) and pass with the fix applied.

Ran the full suite: npm test → 210 test files / 4719 tests passed, 25
skipped (pre-existing skips, unrelated to this change). npm run build and
npm run typecheck both clean.