feat(cron): write execution summary to agent memory after job completion

[kagura-agent]

Problem

When cron jobs run, they execute in isolation. The main conversational agent has no knowledge of what cron jobs did. Users ask "did you send me a briefing?" and the agent says "no" even though it did.

Solution

After a cron job completes successfully, write a lightweight record to the agent's MEMORY.md via the existing MemoryStore mechanism.

Entry format

[Cron] daily-briefing ran at 2026-04-03 09:00 — Weather is sunny, 22°C. Top news: ...

• [Cron] prefix helps the agent distinguish cron records from its own notes
• Summary truncated to 150 chars to stay within memory budget
• Respects the existing § delimited entry format (handled by MemoryStore.add())

Opt-out configuration

Two levels of opt-out, both defaulting to enabled:

1. Per-job: set memory_writeback: false in the job definition
2. Global: set cron.memory_writeback: false in config.yaml

Safety

• Only writes for successful jobs (failures are already logged to output files)
• Memory write errors are caught and logged at debug level — never crashes the tick loop
• If memory is full (MemoryStore.add() returns an error), it's silently ignored

Changes

File	What
cron/scheduler.py	Added _write_memory_summary() function; called in tick() after save_job_output
cron/jobs.py	Added memory_writeback parameter to create_job()
tests/cron/test_scheduler.py	9 new tests for memory writeback behavior
tests/cron/test_jobs.py	3 new tests for memory_writeback field persistence

Tests

All 99 cron tests pass:

pytest tests/cron/ -q
99 passed, 4 skipped in 3.62s

Closes #2704

[kagura-agent]

CI Note

The 5 test failures are pre-existing and unrelated to this PR:

Failed Test	Reason	Related to this PR?
test_parallel_mixed_approve_deny	Gateway approval logic	❌ No
test_gateway_run_agent_codex_path_handles_internal_401_refresh	Codex execution path	❌ No
test_content_under_limit_passes	Timeout (30s)	❌ No
test_oversized_read_rejected	Timeout (30s)	❌ No
test_custom_config_raises_limit	Timeout (30s)	❌ No

Verification: The same test job also fails on the most recently merged PR (#4660) with the same pattern. Our changes only touch cron/ files — all 99 cron-related tests pass locally and in CI.

[kagura-agent]

Gentle ping — any feedback on this? Happy to adjust the approach. 🙏