Document that Linear archives issues automatically

[schpetbot]

Follow-up to #285 / #284.

issue archive exposes an action Linear's own product hides. Linear's delete and archive issues docs say "archiving happens automatically with no option to manually archive items", and the 2021 changelog explains why the manual option was removed: most people used the archive as a trash can, and "the archive is something that Linear should manage for you while deleting issues is your own choice". The official Linear MCP server has no archive tool either. Nothing in our help or skill said any of this, so an agent reading the command list would treat archive as a normal way to retire an issue.

Changes

• issue archive --help now explains the caveat and links the Linear docs. The generated skill reference and help snapshot follow from it.
• docs/usage.md quotes and links the docs and changelog, and notes the practical hazards of calling issueArchive directly: it skips auto-archive's checks for open parents, sub-issues, cycles, and projects, and archived issues drop out of list, query, and search unless --include-archived is passed.
• The skill template gains a "Close, delete, or archive an issue" recipe that tells agents to prefer closing or deleting, and to archive only when explicitly asked.
• README comment and changelog entry.

Verification

• deno check, deno lint, deno fmt --check on changed files
• deno task test test/commands/issue/issue-archive.test.ts: 11 passed
• deno task generate-skill-docs and deno task snapshot for the regenerated reference and help snapshot