fix: don't abort semver set when a post_version hook's file is missing

[justinmchase]

Summary

• Fixes post_version hook aborts entire 'semver set' run when hook target file is missing #171
• A post_version hook (replace/patch/regexp) whose target file doesn't exist threw an uncaught Deno.errors.NotFound out of postVersionHook, aborting the whole semver set/semver inc * run even though the VERSION file write and any earlier hooks had already succeeded.
• Missing hook files are now treated as recoverable warnings: the hook is skipped, remaining hooks still run, and the process still exits 0.
• Warnings are collected and surfaced via the existing GITHUB_OUTPUT mechanism (writeGithubOutput in src/util/version.ts) as a new hook_warnings field (JSON array of { kind, file, reason }), so callers can detect and react to them (e.g. to leave a commit comment).
• Other hook error types (unknown kind, invalid config, etc.) still fail fast as before — this only relaxes the "target file doesn't exist" case.

Test plan

• deno fmt --check
• deno lint
• deno test (CI — this environment's network policy blocks direct registry.npmjs.org access needed to fetch npm deps, so it couldn't be run locally; added new cases to src/hooks/post.test.ts covering: a hook with a missing file resolves with a warning instead of rejecting, and remaining hooks still execute after one fails)