chore: adopt linecheck for per-file line-count enforcement

[tupe12334]

Closes #183

What

• Add linecheck.yml at the repo root: **/*.rs warns at 200 lines, errors at 1100 lines.
• Add a linecheck job to .github/workflows/ci.yml (installs via cargo install linecheck --locked, runs linecheck .).

Why 1100 and not the tool's default 400?

core/src/run.rs is currently 1000 lines. Gating at the tool default (400) would fail every PR immediately. Starting the error threshold at 1100 (buffer above the current max) keeps CI green today while still catching new runaway growth and surfacing every file over 200 lines as a non-blocking warning. Lower the gate in follow-up PRs as files get split — see moadim-io/daemon's history for the iterative pattern.

Verification

Ran locally against this repo:

$ linecheck .
./Architecture.md: 228 lines (warn threshold: 200)
./core/tests/gate_integration.rs: 247 lines (warn threshold: 200) — Getting long — consider splitting into submodules
./core/tests/cli.rs: 224 lines (warn threshold: 200) — Getting long — consider splitting into submodules
./core/src/flow.rs: 438 lines (warn threshold: 200) — Getting long — consider splitting into submodules
./core/src/bin/main.rs: 532 lines (warn threshold: 200) — Getting long — consider splitting into submodules
./core/src/cel_eval.rs: 465 lines (warn threshold: 200) — Getting long — consider splitting into submodules
./core/src/run.rs: 1000 lines (warn threshold: 200) — Getting long — consider splitting into submodules
$ echo $?
0

All warnings, no errors — the new CI job will pass.