feat: add gemara-bundle skill for bundle manifest creation

[sonupreetam]

Summary

• Adds skills/gemara-bundle/SKILL.md — guides LLMs through creating bundle manifests with correct format
• Prevents a common LLM hallucination where object-style entries (- path: ...) are used instead of plain strings (- ...), which breaks the publish workflow's discover step
• Follows the same SKILL.md pattern as feat: adds claude plugin for gemara mcp and skills #6 (frontmatter, prerequisite scan, wizard steps, format rules)

Naming

The skill uses the gemara- prefix for consistency with other skills in this plugin (gemara-policy, gemara-artifact-authoring, etc.), even though bundle manifests do not have a corresponding CUE schema definition — they are a CI convention for the publish workflow.

Context

Observed CI failure in an exercise repo where the LLM generated:

layers:
  - path: governance/policies/container-security.yaml

instead of:

layers:
  - governance/policies/container-security.yaml

The bundle manifest is a CI convention (not a Gemara schema type), so it doesn't belong in gemara-mcp. This skill provides the "last mile" context after artifacts are authored — wiring them for OCI publishing.

Depends on

• feat: adds claude plugin for gemara mcp and skills #6 (establishes the skills/ directory and plugin structure)
• Part of Create a Claude Marketplace Plugin #5

[sonupreetam]

As PR #6 is merged, @hbraswelrh & @jpower432, this PR is ready for review. This from the UAT testing to prevent LLM hallucination.

[slegarraga]

The skill reads well and the core rule is clear: plain string paths, no key-value entries, relative to the repo root, and the last entry is the root artifact. That directly addresses the hallucination failure described in the PR. One optional follow-up would be linking to an existing bundle manifest or the discover step so the contract is discoverable from the skill itself. No blocking issues from my side.