feat(mcp): follow-up-issue action spec builder for deferred review findings

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The follow-up-issue slice of #1962: build a LocalWriteActionSpec that files a follow-up issue for a deferred finding, composing the issue title/body from the finding. Reuses the existing buildFileIssueSpec pattern — gittensory supplies content, the contributor runs it locally with their own gh creds (boundary-safe). No new MCP write path.

Deliverables

• New buildFollowUpIssueSpec in src/mcp/local-write-tools.ts composing title/body from a review finding and delegating to the file_issue spec shape
• Public-safe title/body assembly (bounded length; strip markers)
• Default label wiring so the follow-up carries a point-bearing label when supplied (e.g. gittensor:bug)
• Tests: spec shape, POSIX single-quote escaping via sq (src/mcp/local-write-tools.ts:22), label args, empty-label branch

References

• src/mcp/local-write-tools.ts:42
• src/mcp/local-write-tools.ts:22
• src/mcp/server.ts:1394

Part of #1962.

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2278