feat(tools/mercurial): Mercurial (Hg) version-control support for the local dev-loop

[potiuk]

Add tools/mercurial/ so the local dev-loop skills can operate on Mercurial (Hg) repositories, not just Git. Today the framework assumes Git everywhere — git checkout/commit/diff/rebase are baked directly into issue-fix-workflow, pr-management-code-review, and the setup-steward worktree machinery. This is the first "not only Git" version-control bridge and should establish the abstraction the others (SVN, Jujutsu, Fossil, Perforce) reuse.

Capabilities to provide (mirror the implicit Git contract the skills rely on):

• Working-copy ops: branch/bookmark create + switch, commit, hg diff, hg status
• History read: log, annotate/blame, file-at-revision (for the reproducer + reassess skills)
• Patch/queue export for the fix-workflow hand-off (hg export, optionally MQ/evolve)
• A thin VCS-capability shim so skills call vcs.diff / vcs.commit rather than git ... directly

Why: Mercurial still backs a meaningful slice of OSS — the Mercurial project itself, large historical Mozilla trees, Octobus/Heptapod (GitLab-on-Hg) hosting, Meta's internal Sapling lineage, and many academic / scientific repos. A project-agnostic stewardship framework that hard-codes Git can't be adopted by an Hg shop at all.

Reference:

• Mercurial CLI: https://www.mercurial-scm.org/doc/hg.1.html
• Heptapod (GitLab + Hg): https://heptapod.net/
• Git touchpoints to abstract: search the skills for git checkout / git commit / git diff / <default-branch>