fix: unbreak main, and add the CI that would have caught it

[stormer78]

main does not compile.

error[E0223]: ambiguous associated type
   --> src/config.rs:280:23
    |
280 |             identity: Identity::PnmSession {

#7 and #8 both edited the test module in src/config.rs, from branches that did not know about each other. Git merged them cleanly — the blocks are textually distinct — but #7's test references Identity::PnmSession, an enum variant #8 deleted. A textual merge cannot see that; only building the result can.

Nothing built the result. This repo has shipped eight PRs with no CI at all, only a DCO check.

It stayed quiet because the breakage is confined to the test target: cargo build still succeeds, so anyone cloning to use the plugin would not notice, and anyone cloning to work on it would trip over it immediately with no way to tell it was pre-existing rather than their own doing.

The fix

Removes the stale test — #8 already covers its intent with the_sessions_dir_is_never_left_to_a_default — and adds CI.

The CI, and why it looks like this

• fmt, clippy, test under RUSTFLAGS=-D warnings.
• On macOS, deliberately. Two of the bugs shipped in this repo were platform-specific paths — pnm stores config under dirs::config_dir(), which is ~/Library/Application Support on macOS and ~/.config on Linux. A Linux-only runner would have been green through both.
• On merge_group as well as pull_request, because "a PR that was green on its own branch" is exactly what just broke main. Checking only the branch would not have caught this.
• A manifest job. The plugin JSON files and the command/skill frontmatter are what a person meets before any Rust runs; a malformed plugin.json breaks the plugin without breaking the build, so the build alone would never report it.

Every step was dry-run locally before being committed — a CI file that has never executed is a guess, and this PR is about not guessing.

Testing

cargo test — 84 pass. cargo clippy --all-targets --all-features clean under -D warnings. cargo fmt --check clean. Both CI jobs' commands run locally.