Stacked on #187.

### Rust workflow steps

`flint init` now adds `Swatinem/rust-cache` and `rustup component add clippy
rustfmt` to `.github/workflows/lint.yml` when a Rust toolchain is detected in
`mise.toml`. The push trigger gets a comment explaining why we run on main
("warms the Rust cache so PR branches get a cache hit").

Also bumps the generated `mise-action` pin to v2026.4.16 to match flint's
own dogfood workflow.

### Markdownlint .yml migration

Switches the generated markdownlint config from `.markdownlint.jsonc` to
`.markdownlint.yml` for uniformity across consumer repos. Legacy variants
(`.markdownlint.{json,jsonc,yaml}` and `.markdownlint-cli2.*`) are replaced
on init. Only generated when editorconfig-checker is also selected, since
the file's comment points to editorconfig-checker for line-length.

### [tools] normalization

New `normalize_tools_section` sorts `mise.toml [tools]` alphabetically and
inserts a `# Linters` header. Toolchain keys (rust/go/dotnet/node) stay
above the header; lint-only binaries go below.

Toolchain detection is registry-driven: a new `Check::toolchain()` /
`Check::toolchain_components()` builder marks a mise_tool as a language
runtime. `toolchain_keys()` collects them at runtime, plus `node` (which
is added by `ensure_node_for_npm` outside the registry).

The old `mise_install_components: Option<&str>` field merges into
`toolchain: Option<Option<&str>>` — None = linter binary, Some(None) =
toolchain without components, Some(Some(c)) = toolchain with components.

### Idempotence fix

Before: first run generates `.github/workflows/lint.yml` but doesn't
detect actionlint (no workflow yaml existed at detect time). Second run
picks up the generated workflow → adds actionlint → non-idempotent.

Fix: when init is about to generate the workflow, synthesize the workflow
patterns into `present_patterns` so actionlint gets selected in the same
run. Covered by new `tests/cases/general/init-idempotent/` e2e case.

### Tests

- New e2e cases: `general/init-rust` (fresh rust project → full init),
  `general/init-idempotent` (fully-initialized repo → "No changes to apply")
- init-rust test uses a fake `mise` shell-script stub to deterministically
  pin tool versions, via the existing `[fake_bins]` harness feature
- Fixes a snapshot-writer bug in `write_test_toml` that was producing
  malformed test.toml (missing newlines, escape-processing strings)
- New unit tests for `normalize_tools_section`, markdownlint migration,
  rust workflow generation
Signed-off-by: Gregor Zeitlinger <[email protected]>

Adds `github:grafana/flint` at the running binary's CARGO_PKG_VERSION so
all contributors run the same flint release. Skips when the key already
exists (any pin) to respect user's explicit choice.

Signed-off-by: Gregor Zeitlinger <[email protected]>