feat(update): add self-update command with stable/beta channels

[aJesus37]

Summary

• Adds hctf update subcommand that checks GitHub Releases and atomically self-replaces the binary
• Supports stable (default) and beta (pre-release) channels via --beta flag or update_channel in config
• Permission check before replacing: re-execs with sudo on Linux if needed, errors with admin message on other OSes

Flags

• --check — check for updates without installing
• --beta — use beta channel (includes pre-releases) for this run
• --yes/-y — skip confirmation prompt

Test Plan

• Unit tests for all helpers (fetchLatestRelease, findAsset, resolveChannel, downloadAndExtract, atomicReplace, canWriteExec, buildSudoArgs)
• Integration test: hctf update --check exits 0 and prints sensible output
• All 8 packages pass (go test -count=1 -timeout 120s ./...)
• Manual smoke test: --help, --check, --check --beta all work

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