feat(init): support CLERK_SKILL_SOURCE env override

[wyattjoh]

Summary

Adds a CLERK_SKILL_SOURCE env var as an escape hatch for skill authors iterating on the clerk skill without rebuilding the CLI. When set, clerk init passes the value straight to <runner> skills add <value> in place of the bundled source (#126).

Any value the skills CLI accepts works:

# Absolute path to a working-tree skill dir (default symlink install,
# edits to the source are reflected in the installed skill immediately).
CLERK_SKILL_SOURCE="$PWD/skills/clerk" clerk init

# A fork or PR branch on GitHub.
CLERK_SKILL_SOURCE="https://github.com/me/cli/tree/wip/skills/clerk" clerk init

# Shorthand for the default repo (installs from main branch).
CLERK_SKILL_SOURCE="clerk/cli" clerk init

The override path skips the staged temp dir and the --copy flag introduced in #126, so a local working-tree path installs via the default symlink mode and edits to the source are picked up by already-installed projects without re-running clerk init. init logs the value being used so there's no surprise about what got installed.

The override has no effect on the upstream framework-pattern skills.

Stacked on #126.

Test plan

• bun run test passes (unit tests cover env-var precedence: unset, empty/whitespace, trimmed / remote / local variants)
• Manual: CLERK_SKILL_SOURCE="$PWD/skills/clerk" bun run dev -- init in a sandbox, confirm the installer logs the override and .claude/skills/clerk/ is a symlink into the working tree
• Manual: edit skills/clerk/SKILL.md in the working tree, re-open the installed skill, confirm the edit is visible without re-running init
• Manual: CLERK_SKILL_SOURCE="clerk/cli" bun run dev -- init installs from the remote shorthand and the upstream framework skills still install normally

[wyattjoh]

Stack: init-skills

• feat(lib): add NonEmptyArray helper, tighten preferredRunner #125
• feat: Clerk CLI Skill #126
  • feat(init): support CLERK_SKILL_SOURCE env override #127 👈 this PR
  • docs(skill): add audit-clerk-skill for drift detection #156

Part of a stacked PR chain. Do not merge manually.

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🦋 Changeset detected

Latest commit: e6727c1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
clerk	Minor

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