fix(scheduler): accept 5-field cron by padding seconds 0 (ADR-0021)

[chinkan]

Closes #155. Implements ADR-0021 (docs/adr/0021-five-field-cron-seconds-pad.md). Timezone is out of scope (#166); crons are still evaluated in UTC.

Change

• agent::normalize_cron_expr replaces validate_cron_expr: 5 fields → "0 " + fields; 6 fields pass through; 4/7 fields and @macros are rejected with a clear error.
• Portal POST /api/tasks and PUT /api/tasks/{id} store the normalised 6-field value and echo it (triggerValue); 4/7 fields still return 400 invalid_cron.
• schedule_task tool stores the normalised value and shows it in the success text; the trigger_value description now documents 5 or 6 fields, UTC, stored as 6.
• Built-in [memory] summarize_cron / [learning] user_model_cron are normalised at boot; errors name the config key (still fail-fast).
• Boot restore: ScheduledTaskStore::normalize_stored_cron rewrites legacy 5-field rows (info log Normalised 5-field cron for task …), leaves 6-field rows untouched, and never arms an invalid row (error log).
• Portal: CRON_5_OR_6_FIELD heuristic; en / zh-HK copy says 5 or 6 fields with a 5-field example.

Tests

• Unit: normalize pads 5 / passes 6 / rejects 4, 7, @daily; padded 0 9 * * * → next 09:00:00 UTC; restore heal (5 rewritten, 6 untouched, 7 refused, row unchanged).
• Integration: portal POST 5-field stored + echoed; PUT 5-field stored, 4-field 400; tool 5-field stored + shown.
• Web: vitest accepts 5/6, rejects 4/7.
• cargo fmt --check, cargo clippy --all-targets -D warnings clean; cargo test: 929 passed; web: 41 passed.

QA matrix

1. Portal and Telegram tool: 0 9 * * * → stored 0 0 9 * * *, fires 09:00 UTC (17:00 HKT).
2. An existing 5-field row in the DB is healed at boot (log line + row rewritten) and fires.
3. 9 * * *, 0 0 9 * * * 2026, @daily are rejected (portal 400 invalid_cron, tool error).
4. Portal form accepts a 5-field cron; hint/labels say 5 or 6 fields.

[chinkan]

TL ACCEPT on tip 45cee9a (ADR-0021 / #155).

Verified: 5→6 seconds pad, 6 unchanged, reject 4/7/@macros, portal POST/PUT + tool + config crons store normalised 6-field, restore rewrites 5-field rows, portal en/zh-HK UTC hint, schedule_task already has Evaluated in UTC., no timezone behaviour change (#166 separate).

Non-blocking follow-ups (not this PR): rename triggerLooksValid per ADR §6; a few stale "6-field" docs/comments/placeholder; restore test could assert full arm path.

Waiting on Product GO + QA GO + CI green before merge. No tag.