ci: add InTx linter replacing ruleguard rule (#24422)

Replace the old `InTx` ruleguard rule in `scripts/rules.go` with a
custom in-tree `go/analysis` analyzer under `scripts/intxcheck/`. The
new analyzer catches the same direct and pass-through misuse classes as
before, plus two new classes the pattern-matcher couldn't reach:

- **Indirect same-package helper misuse** — flags `p.someHelper(ctx)`
inside `InTx` when the helper body uses the outer store (the PR #24369
bug class).
- **Nested dangerous closures** — descends into `go func() { ... }()`,
`defer func() { ... }()`, and immediately-invoked function literals.

The analyzer uses semantic `types.Object` identity instead of raw
expression string comparison, which avoids false positives from
closure-local shadowing and catches simple aliases like `outer := s.db`
and `alias := s`.

This PR also fixes three real outer-store-inside-transaction bugs the
new analyzer surfaced:

- `coderd/wsbuilder/wsbuilder.go`: `FindMatchingPresetID` and
`getWorkspaceTask` now use the inner transaction store instead of
`b.store`.
- `enterprise/dbcrypt/dbcrypt.go`: `ensureEncrypted` now calls
`s.InsertDBCryptKey` (the tx-wrapped store) instead of
`db.InsertDBCryptKey`. The `dbCrypt.InTx` method wraps the raw tx in a
new `*dbCrypt`, so `s.InsertDBCryptKey` still dispatches through the
encryption layer.

Two call sites need `// intxcheck:ignore` suppressions. Both are one-off
patterns that only look like misuse because the analyzer doesn't track
assignments — proving them safe would require full dataflow analysis,
which is well beyond what a targeted lint like this should attempt:

- `coderd/database/dbfake/dbfake.go` — `b.db` is reassigned to `tx` on
the preceding line, so `b.doInTX()` actually uses the transaction. The
analyzer sees the original `b.db` identity and flags it.
- `coderd/database/db_test.go` — test intentionally passes the outer
store to `require.Equal` to assert that nested `InTx` returns the same
handle.

Suppressions use `// intxcheck:ignore` instead of `//nolint:intxcheck`
because `intxcheck` runs as a standalone `go/analysis` tool outside
golangci-lint. golangci-lint's `nolintlint` checker flags `//nolint`
directives for linters it doesn't control, so we use a custom comment
prefix to avoid that conflict.

Makefile

@@ -721,6 +721,7 @@ lint/go:
 	linter_ver=$$(grep -oE 'GOLANGCI_LINT_VERSION=\S+' dogfood/coder/ubuntu-26.04/Dockerfile | cut -d '=' -f 2)
 	go run github.com/golangci/golangci-lint/cmd/golangci-lint@v$$linter_ver run
 	go tool github.com/coder/paralleltestctx/cmd/paralleltestctx -custom-funcs="testutil.Context" ./...
+	go run ./scripts/intxcheck ./...
 .PHONY: lint/go
 
 lint/examples: | _gen/bin/examplegen