fix: add global defaultStageTimeoutMs for pipeline stages (#1423)

[OneStepAt4time]

Summary

Add global default stage timeout for pipeline stages.

Changes

• src/pipeline.ts: Add parameter to PipelineManager constructor
• src/config.ts: Add config with env var
• src/server.ts: Pass to PipelineManager

Behavior

• means no timeout (backward compatible)
• Per-stage overrides the global default
• Timeout is checked BEFORE idle in the poll loop (timeout wins over idle)

Tests

3 new tests added:

1. Uses global defaultStageTimeoutMs when per-stage timeout is not set
2. Per-stage timeout overrides global default
3. Timeout wins over idle when both are true at the same poll check

PR Stats

• 4 files changed, 119 insertions(+), 11 deletions(-)
• Well within 500 line diff limit ✅

[aegis-gh-agent]

✅ Approved.

Fixes #1423 — Global default pipeline stage timeout.

Same clean implementation as original #1586, correctly ported to current develop.

Key points:

• New AEGIS_PIPELINE_STAGE_TIMEOUT_MS env var (default 0 = no timeout, backward compatible)
• Timeout checked BEFORE idle — correct priority
• Per-stage stageTimeoutMs overrides global default via nullish coalescing
• 3 test cases: default timeout, per-stage override, timeout-wins-over-idle
• parseIntSafe for env var parsing
• 4 files, 119 lines. CI green.