Consolidate repo guidance into AGENTS.md

[xingyaoww]

Summary

• Move content from .openhands/microagents/repo.md to AGENTS.md
• Create symlink .openhands/skills/repo.md -> ../../AGENTS.md
• Create symlink CONTRIBUTING.md -> AGENTS.md (if not existing)

This establishes AGENTS.md as the single source of truth for repository guidance, following the pattern from OpenHands/OpenHands-CLI#340

[enyst]

Hi! I'm OpenHands (automated agent) helping with repo work.

Heads up: this PR introduces a legacy-path symlink .openhands/skills/repo.md -> ../../AGENTS.md.

We found that keeping this .openhands/skills/repo.md symlink causes the instructions to be loaded twice (AGENTS.md + the legacy repo skill path), which duplicates the guidance.

CONTRIBUTING.md -> AGENTS.md is fine; the duplication issue is specifically the .openhands/skills/repo.md symlink.

Example/fix: OpenHands/software-agent-sdk#1709 (removes the .openhands/skills/repo.md symlink).

[xingyaoww]

Addressed review feedback: removed legacy-path symlinks pointing to AGENTS.md (they caused instructions to be loaded twice when both AGENTS.md and the symlink target were loaded).

Replaced the symlinks with small pointer files:
- `CONTRIBUTING.md` (was symlink to `AGENTS.md`)

• .openhands/skills/repo.md (was symlink to ../../AGENTS.md)

[xingyaoww]

Follow-up per review: removed legacy guidance files so only AGENTS.md remains.

Removed:
- `.openhands/skills/repo.md`

• CONTRIBUTING.md