chore(issue-template): add migrate issue template

[Viktor (tiulpin)]

Summary

Split out of #226. Adds the GitHub issue form for reporting teamcity migrate conversion problems, so bug reports arrive with the source CI file, generated .tc.yml, and command output already structured.

Changes

• Add .github/ISSUE_TEMPLATE/migrate_issue.yml (labels migrate, bug; both already exist in the repo).

Design Decisions

Straightforward.

Example

N/A — not user-visible (GitHub issue form).

Test Plan

• Unit tests pass (just unit) — N/A — no code change
• Linter passes (just lint) — N/A — no code change
• Acceptance tests pass (just acceptance) — N/A — no code change
• If adding a new command/flag: added .txtar test in acceptance/testdata/ — N/A
• If adding a data-producing command: includes --json support — N/A
• If modifying --json output: no field removals/renames (additive only) — N/A
• If changing docs-visible behavior: updated docs/, skills/, and README.md — N/A — issue template only
• External contributors: links a status:finalized issue — N/A

[chatgpt-codex-connector]

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Reviewed commit: 9e1ce220d5

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[chatgpt-codex-connector]

Do not publish a template for absent migrate tooling

This issue form is user-facing as soon as it lands, but the reviewed tree does not register a teamcity migrate command in internal/cmd/root.go and repo-wide search only finds migrate-to-teamcity in this new template. That means users opening the issue chooser are directed to report/run tooling that this release cannot provide; either add the migrate command/bundled skill in the same change or keep this template out until that feature exists.

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