fix: respect explicit mode input instead of always auto-detecting

[FuturizeRush]

Summary

When users set mode: agent in their workflow, the action ignores it and exits with "No trigger found, skipping." This PR makes the explicit mode input actually work.

Fixes #1197

Root cause

1. action.yml:246 sets MODE=${{ inputs.mode }} as an env var
2. detectMode() in src/modes/detector.ts never reads process.env.MODE — it only auto-detects from event context
3. Trigger check (run.ts:194-197) requires context.inputs.prompt to be truthy for agent mode, but users using direct_prompt have an empty prompt → containsTrigger = false → skip

process.env.MODE is set but never read anywhere in the codebase.

Fix

Two changes in src/entrypoints/run.ts:

1. Check process.env.MODE before calling detectMode() — if explicitly "agent" or "tag", use it directly instead of auto-detecting
2. Bypass trigger detection when mode is explicit — the user opted in by setting mode, so no trigger check is needed

What changes

Scenario	Before	After
mode: agent + direct_prompt	"No trigger found, skipping"	Runs Claude
mode: agent + prompt	"No trigger found, skipping" (if no entity context)	Runs Claude
mode: tag (explicit)	Auto-detected as tag (worked by coincidence)	Explicitly tag
No mode set	Auto-detect	Auto-detect (unchanged)

Test plan

• TypeScript compiles with zero errors (npx tsc --noEmit)
• Traced the full code path: action.yml:246 → process.env.MODE → run.ts:156 → detectMode() confirms MODE is never read
• Confirmed base-action/src/index.ts already handles mode correctly (this fix aligns run.ts with that behavior)