Add 5 new platforms via declarative spec registry (#80)

[devrimcavusoglu]

Closes #80.

Summary

• Replace per-platform Go files (claude.go, codex.go, opencode.go) with a single declarative Spec table in internal/platform/spec.go and one generic Adapter struct in internal/platform/adapter.go. Adding a platform is now a one-line append to Specs plus one Type constant — no new Go file required.
• Ship 5 new adapters: cursor, gemini-cli, github-copilot, windsurf, continue. Path conventions follow vercel-labs/skills.
• Detection is per-platform, not per-directory. Several adapters share .agents/skills/ as their project dir; each one's DetectHome keys on a distinct user-level config dir (~/.cursor, ~/.gemini, ~/.copilot, ~/.codex, etc.) so platform list doesn't false-positive for agents whose CLI isn't installed.
• CLI flag help and the "unknown platform" error text enumerate the spec registry dynamically — adding a platform updates user-facing text without further code changes.
• Tests are table-driven over Specs: install/list/uninstall round-trips run for every registered platform, plus a dedicated TestAdapter_SharedProjectDir that locks in the shared-.agents/ semantics.

Acceptance criteria (from #80)

• At least 5 new platforms supported: cursor, gemini-cli, github-copilot, windsurf, continue.
• Adding a new platform = one PR touching one data file: append to Specs in internal/platform/spec.go plus a Type constant. Documented in docs/concepts/platform-adapters.md.
• No false positives on shared .agents/skills/: detection uses each platform's own user-level config dir, covered by TestAdapter_Detect_FallbackPaths and the per-platform Detect tests.
• Capacity behavior in shared-directory case: design is recorded in docs/concepts/platform-adapters.md and the CHANGELOG. Threshold protects the directory, not the logical agent — when two adapters read the same dir, both see the same count.
• No breaking change for existing claude-code / codex-cli / opencode users — all three keep their original Name() and on-disk paths. codex-cli keeps ~/.agents/skills/ (vercel uses ~/.codex/skills/) so existing installs are untouched; the detector now also checks ~/.codex/ as a positive signal.

Implementation notes

• internal/platform/spec.go — Spec struct + the Specs registry. Source of truth.
• internal/platform/adapter.go — single generic Adapter implementing the Platform interface from any Spec.
• internal/platform/detector.go — NewDetector builds adapters from Specs; ParsePlatformType validates against the registry; error messages list every registered name.
• internal/cli/capacity.go — adds platformNamesList() shared by skill install, skill uninstall, and skill diff so flag help and error text stay in sync with the registry.
• Tests in internal/platform/platform_test.go walk every Spec for install/uninstall/detect — replaces ~250 lines of per-platform duplication.

Test plan

• make test — full suite passes
• make lint — golangci-lint reports 0 issues
• make test-smoke — 57/57 smoke tests pass
• ./skern platform list on real host — shows all 8 platforms with correct detection state
• ./skern skill install --help — flag help dynamically lists the 8 platforms

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