`gonext doctor` — verify a scaffolded project still matches its guardrails

Nothing today tells a developer that their generated project has drifted out of the shape gonext set up for it. Candidate checks, run from inside the project: mise installed and matching the versions pinned in `.mise.toml`, lefthook actually installed into `.git/hooks`, agent files coherent (an `AGENTS.md` present, no pointer file orphaned without it), the Makefile targets the guardrails cite existing, and possibly every registered route having a matching `.bru` file. Output is a pass/fail list with a fix hint per failure.

Merged in from #15's `gonext doctor` row, which described the same command as a fast local health check before starting work — two further checks belong on the same list: the database is reachable (what `make db-up` brings up), and generated files are not stale (`wire_gen.go` vs. the DI providers, the same drift `make generate-check` catches).

This sits on a different axis from `make check` (#49): that one validates the code — lint, vet, test — while this validates setup and guardrail coherence. It is CLI-native rather than scaffolded, which is the point: it improves for every project already out there without anyone re-scaffolding, something a change to `templates/` can never do.

Came out of an ideation session around #13, not a scoped request. Now a sub-issue of #15.

Start by reading the existing CLI entry points and the scope of #15, then inspect make generate-check and the mentioned guardrails: .mise.toml, AGENTS.md, Makefile, wire_gen.go, and database setup. Define the checks and fix hints before implementing gonext doctor; done means it reports each selected setup or drift condition as pass/fail.

Written by the indexing model from the issue text.