fix(prime): suppress spurious session_start on bare gt prime calls

[mk-imagine]

Problem

gt prime unconditionally emitted a session_start event via emitSessionEvent
inside setupPrimeSession, even when called without --hook. Any agent calling
gt prime <other-agent-dir> to read context would emit a spurious session_start
using the target agent's persisted session_id.

Observed symptom: Boot's formula calls cd ~/gt/deacon && gt prime to read
the Deacon's context. This caused every Boot triage cycle to emit two session_start
events — one for Boot itself (correct) and one for the Deacon (spurious), using the
Deacon's stored session_id from .runtime/session_id. Effects:

• Polluted gt seance discovery with phantom sessions
• Made the town event feed misleading (appeared as double spawning)
• Made token cost attribution harder to audit

Fix

Guard emitSessionEvent behind primeHookMode in setupPrimeSession. Session
events are only emitted when gt prime is genuinely running as a SessionStart
or PreCompact hook (--hook flag set). Bare gt prime reads are unaffected
functionally.

Testing

With Gas Town running, observed ~/gt/.events.jsonl across multiple Boot triage
cycles. Before fix: two session_start events per cycle (one with Boot's session_id,
one with Deacon's). After fix: exactly one session_start per cycle.

Note

Pre-existing go vet failure in internal/cmd/compact_report_test.go:29
(wispTypeToCategory argument count mismatch) exists on main and is unrelated
to this change.

[Copilot]

Pull request overview

Suppresses unintended session_start emission when gt prime is invoked without --hook, preventing phantom sessions from being logged when one agent primes another agent’s directory for context reads.

Changes:

• Guarded emitSessionEvent(ctx) behind primeHookMode in setupPrimeSession.
• Added explanatory inline comment documenting why non-hook gt prime must not emit session_start.

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