feat: add Bitbucket Cloud support

[devtedlee]

Summary

• Add full Bitbucket Cloud (bitbucket.org) support with feature parity to GitHub/GitLab
• Users can now skills add, skills check, and skills update for Bitbucket-hosted skills
• Supports private repos via BITBUCKET_TOKEN or BITBUCKET_USERNAME + BITBUCKET_APP_PASSWORD environment variables

Changes

File	Description
src/types.ts	Add 'bitbucket' to ParsedSource type union
src/source-parser.ts	Add 3 Bitbucket URL regex patterns, exclude from isDirectSkillUrl / isWellKnownUrl
src/skill-lock.ts	Add getBitbucketAuthHeaders() and fetchBitbucketSkillFolderHash()
src/cli.ts	Update runCheck() / runUpdate() to support Bitbucket, add help text example
src/add.ts	Add Bitbucket branches to hash fetching in 2 locations
README.md	Add Bitbucket URL example to Source Formats
tests/source-parser.test.ts	Add 7 Bitbucket URL parsing + getOwnerRepo tests

Design Decisions

• Hash detection: Bitbucket lacks GitHub-style tree SHAs, so the latest commit hash affecting the skill folder path is used as a change-detection mechanism via GET /2.0/repositories/{workspace}/{repo}/commits?include={branch}&path={path}&pagelen=1
• URL pattern: Bitbucket uses /src/branch/path (vs GitHub /tree/ and GitLab /-/tree/)
• Auth: Supports Bearer token (BITBUCKET_TOKEN) and Basic auth (BITBUCKET_USERNAME + BITBUCKET_APP_PASSWORD); unauthenticated access works for public repos

Test Plan

• All existing unit tests pass (64/64)
• New Bitbucket URL parsing tests pass (5 parseSource + 2 getOwnerRepo)
• E2E: skills add from private Bitbucket repo — clones and installs skill
• E2E: skills check — detects no updates when up to date
• E2E: Push change to Bitbucket → skills check detects update
• E2E: skills update — applies update, verified file content reflects changes
• E2E: skills check after update — confirms "All skills are up to date"

Closes #277