An issue body's `Files:` line, when present, is the only list of files the hot-file hold reads #6475

Today expectedFiles in src/crew/hot_files.ts takes every path an issue mentions, including the ones it names only as context ("see src/actions/pulls.ts"). A ready issue can then wait on a hot file it will never touch.

Done:

• [ ] When an issue body has a line that starts with Files:, expectedFiles returns only the paths on that line, in its two existing forms: paths with a directory, and backticked bare names. Without such a line, it behaves exactly as today.
• [ ] The comment at the top of src/crew/hot_files.ts says so, and AGENTS.md tells planners to end an issue with a Files: line.
• [ ] test/hot-files-expected.test.ts covers three bodies: one with a Files: line plus context paths elsewhere (only the Files: paths come back), one without the line (unchanged), and one with an empty Files: line (nothing comes back).

Files: src/crew/hot_files.ts, AGENTS.md, test/hot-files-expected.test.ts