Changes

pipelines(tiflow): increase pipeline timeout and keep only pipeline-level timeout (#4142)

### What changed
- Increased **pipeline-level** `options { timeout(...) }` for TiFlow
pipelines under `pipelines/pingcap/tiflow/latest`.
- Removed all **non-pipeline-level** timeouts:
  - stage-level `options { timeout(...) }`
  - nested `timeout { ... }` blocks
- Updated `AGENTS.md` to include **Development Guidelines → Git Commit
Convention** (Conventional Commits)

### Pipelines updated
- `ghpr_verify.groovy` (40m → 90m)
- `merged_unit_test.groovy` (40m → 120m)
- `pull_cdc_integration_*.groovy` (60–75m → 120m)
- `pull_dm_*.groovy` (60m → 120m)
- `pull_syncdiff_integration_test.groovy` (40m → 120m)
 (commit: 1ab8e2e)