feat(cli): accept Buzz message links for thread reads (#6359)

## Summary

- let `buzz messages thread` accept a canonical `buzz://message` link
directly
- treat the selected message ID as authoritative, verify its channel,
and derive its containing thread root from the event
- reject stale or forged `thread` hints that disagree with the selected
message
- preserve the existing `--channel` / `--event` form while allowing it
to target any message in a thread

## Why

Agent skills should be able to hand first-class Buzz links to the public
Buzz CLI instead of carrying their own URL parsing wrappers. Link
handling belongs with the CLI that owns the protocol; identity and relay
selection remain explicit through the existing Buzz CLI environment and
do not read Buzz Desktop private state.

## Verification

- `cargo test -p buzz-cli` — 360 passed
- `cargo clippy -p buzz-cli -- -D warnings`
- `cargo fmt --all -- --check`
- repository pre-push gates, including 5,070 Desktop tests
- final architecture/code review — no must-fix findings

A relay-backed manual smoke test was not run because this session did
not have `BUZZ_RELAY_URL` and `BUZZ_PRIVATE_KEY` configured; the change
has focused parser, CLI argument, channel/root binding, and mismatch
tests.

---------

Signed-off-by: morgmart <98432065+morgmart@users.noreply.github.com>

crates/buzz-cli/README.md

@@ -34,6 +34,7 @@ buzz messages send --channel <uuid> --content "Reply" --reply-to <event-id> --br
 buzz messages send --channel <uuid> --content - < message.md   # read body from stdin
 buzz messages get --channel <uuid> --limit 20
 buzz messages thread --channel <uuid> --event <event-id>
+buzz messages thread --link 'buzz://message?channel=<uuid>&id=<event-id>&thread=<root-id>'
 buzz messages search --query "architecture"
 buzz messages search --author <pubkey|npub|name> --since <unix-ts>
 buzz messages edit --event <event-id> --content "Updated text"