feat: add bump-version.sh script for OpenCode version updates

[tankdonut]

Summary

• Add scripts/bump-version.sh — a CLI tool to bump the OpenCode version in build/.opencode-version and update corresponding SHA256 checksums in build/.opencode-checksums
• Bumps OpenCode from v1.4.11 → v1.14.18 (checksums verified against GitHub Releases API)

Usage

# Bump to a specific version
./scripts/bump-version.sh 1.14.18

# Auto-detect latest release
./scripts/bump-version.sh --latest

# Preview changes without modifying files
./scripts/bump-version.sh --dry-run 1.14.18

# Show help
./scripts/bump-version.sh --help

What changed

• New file: scripts/bump-version.sh (284 lines, executable)
  • Fetches release metadata from GitHub Releases API (no tarball downloads)
  • Validates semver format, strips v prefix, checks for both x64 + arm64 architectures
  • Atomic file writes via mktemp + mv to prevent inconsistent state
  • Follows repo conventions: [BUMP] prefix, validate.sh color scheme, build.sh arg parsing pattern
  • Prints suggested git add/git commit commands (does not auto-commit)
• Updated: build/.opencode-version → 1.14.18
• Updated: build/.opencode-checksums → v1.14.18 checksums (x64 + arm64)

Verification

All 4 final reviewers APPROVED:

Reviewer	Verdict	Details
Plan Compliance	✅ APPROVE	12/12 Must Have, 10/10 Must NOT Have
Code Quality	✅ APPROVE	ShellCheck clean, conventions match
Manual QA	✅ APPROVE	10/10 scenarios pass (live GitHub API)
Scope Fidelity	✅ APPROVE	No scope creep, spec-complete

Pre-commit hooks passed: shellcheck ✓, trailing whitespace ✓, end-of-file ✓

Test plan

• ./scripts/bump-version.sh --help → exit 0, usage printed
• ./scripts/bump-version.sh 1.14.18 → exit 0, files updated correctly
• ./scripts/bump-version.sh --latest → exit 0, latest version detected
• ./scripts/bump-version.sh --dry-run 1.14.18 → exit 0, files unchanged
• ./scripts/bump-version.sh v1.14.18 → exit 0, v prefix stripped
• ./scripts/bump-version.sh 99.99.99 → exit 1, release not found
• ./scripts/bump-version.sh "not-a-version" → exit 1, semver error
• ./scripts/bump-version.sh (no args) → exit 1, missing version error
• Same-version bump → exit 0, warning + idempotent checksum update
• ./scripts/validate.sh passes after bump