docs(contributing): add review process and issue picking sections (closes #175)

[dezzy-music]

Description

Expands CONTRIBUTING.md with:

1. Picking an Issue: Guidance on selecting issues via Stellar Wave bounty labels, good first issue, and domain area tags, plus the expectation to comment before starting work.
2. Review Process:
   • Expected maintainer response time (24–48 hours on business days).
   • 7-point reviewer checklist (CI status, issue link, PROGRESS.md entry, focused diff, tests, docs, and contract test snapshots).
   • Common reasons for requested changes (missing progress entries, Clippy lints, scope creep, missing tests, secret leaks).

Closes #175

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Tasks Completed

• Added "Picking an Issue" section to CONTRIBUTING.md.
• Added "Review Process" section with response times, reviewer checklist, and common reasons for requested changes.
• Verified all commands, links, and markdown syntax.
• Added progress entry in PROGRESS.md.

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Acceptance Criteria

• All commands and links in the new docs have been verified
• PROGRESS.md entry added, and the PR description includes Closes #175

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Stellar Address (Stellar Wave Program)

GDLCJGQLXZGDAAT5QGRF6FM2MBEER3FPYGCSVPXYL47G2ABIOX2M2S6K