docs: define Phase 3a bounded fleet handling

[aidynamicsolutions]

Summary

• Define the Phase 3a bounded fleet-handling contract in AGENTS.md for routine pickup and heartbeat processing.
• Require the existing canonical bounded fleet snapshot, targeted current-state expansion for actionable or suspicious tasks, authoritative current-state reconciliation, safe incomplete-data handling, duplicate-notification idempotence, and unchanged Phase 3b report behavior.
• Keep the final PR scope to one tracked file: AGENTS.md with 8 insertions and 1 deletion.

This final head is based exactly on 6789876442d0fb6da9f70d86399a2930c5073ae2 and is 74f973a8262a547ed6a3bddea82fd43c93929e46. No token savings are claimed because no matched native-usage comparison was run.

Informational inherited baseline failures

Before any PR change, on the exact base commit, these existing tests failed and are deliberately not repaired in this Phase 3a PR:

• bash tests/fm-bearings-snapshot.test.sh: the mixed secondmate fixture's sed insertion can concatenate the inserted in-flight row with ## Queued on macOS, causing the inherited readiness assertion to fail.
• bash tests/fm-wake-queue.test.sh: the inherited oversized unread-status assertion asks macOS grep to match a 20,000-character pattern and reports grep: out of memory.

These are test-environment or fixture portability issues outside the accepted Phase 3a scope, not runtime regressions from this PR.

Validation

Passed:

• bash tests/fm-fleet-snapshot-view.test.sh
• bash tests/fm-wake-drain-open-decisions-cursor.test.sh
• bash tests/fm-wake-drain-unread-status.test.sh
• bash tests/fm-crew-state.test.sh
• bash tests/fm-bootstrap.test.sh
• bin/fm-lint.sh
• bin/fm-doc-audience-check.sh
• git diff --check

The two inherited baseline failures are recorded above and remain unchanged in the final diff.