fix(reminder): inject system-reminder via context hook, not tool_result

[any-victor]

Closes #20.

Problem

The system-reminder injection currently appends a <system-reminder> text block onto unrelated tool_result content:

// src/index.ts
return {
  content: [...event.content, { type: "text" as const, text: SYSTEM_REMINDER }],
};

This corrupts model-visible transcript semantics for any tool that runs when the reminder is due (read, bash, grep, find, …): the reminder appears as if it were part of the tool's actual output. Real consequences:

• transcript debugging becomes painful (which line came from the tool, which from us?)
• the model gets a false attribution: it sees <system-reminder> 'inside' bash output even though bash never wrote it
• pi's session JSONL ends up with task-management policy text wedged into unrelated tool blobs

tool_result is appropriate for observing or rewriting that tool's own output; it isn't appropriate for injecting policy/task-management reminders unrelated to what the tool produced.

Fix

Use two hooks:

1. pi.on("tool_result", …) for cadence tracking only — never mutates content.
2. pi.on("context", …) to inject the transient reminder into the upcoming LLM call's messages. context fires once before each LLM request (including mid-agent-loop after tool results have been appended), and the returned messages are used only for that one request — not persisted in the session store and not appended to any tool's output.

The reminder is appended as a role: "user" text message so models that don't support custom message types still receive it.

What changes

• src/reminder-cadence.ts (new): pure cadence logic factored out for unit-testability.
  • createCadenceState, resetCadenceState, onTurnStart
  • evaluateToolResult(state, toolName, hasTasks, config)
  • drainReminderForContext(state)
• src/index.ts: hook wiring uses the helpers; tool_result no longer mutates content; new context handler injects when due.
• test/reminder-cadence.test.ts (new): 9 cases covering the decision logic.
• pnpm-workspace.yaml (new): allowBuilds for @google/genai, koffi, protobufjs.

Test plan

pnpm install --no-frozen-lockfile
pnpm run typecheck    # clean
pnpm run lint         # clean
pnpm run test         # 157 / 157 passing across 6 files

Cases pinned in the new test file:

• reminder marks due after REMINDER_INTERVAL non-task turns when tasks exist
• never marks due when no tasks
• never marks due before interval elapses
• task tool usage resets cadence and clears any pending reminder
• no re-fire within the same injection cycle
• re-arms after task tool usage resets cycle
• drainReminderForContext is one-shot per cycle
• resetCadenceState wipes all four fields

Behaviour preserved

• REMINDER_INTERVAL = 4 turns
• task tool usage resets cadence and clears any pending reminder
• never injects when no tasks exist
• never re-fires inside a cycle until a task tool resets it
• session_switch (new/resume) wipes cadence state

[tintinweb]

manually merged see c04674b (master), thanks! 🙏