refactor(compile): replace versioned migrations with detection-based codemods

Aligns with gh-aw's codemod model. The previous design (versioned
chain + `schema-version` field on user front matter) leaked an
internal compiler concern into user files. gh-aw — the project we
explicitly say we're inspired by in AGENTS.md — uses 41 codemods
with no version stamping; users never see a `schema-version` field.

This refactor brings ado-aw to the same model while keeping our
ergonomic compile-time autorewrite UX (gh-aw uses a separate
`gh aw fix --write` command, we don't).

Changes:

- Drop FrontMatter.schema_version field, default_schema_version fn,
  and the SCHEMA_VERSION_KEY const.
- Rename src/compile/migrations/ to src/compile/codemods/. Rename
  src/compile/migration_integration_test.rs to
  src/compile/codemod_integration_test.rs. Rename
  tests/migration_tests.rs to tests/codemod_tests.rs. Rename
  docs/migrations.md to docs/codemods.md (rewritten from scratch).
- Replace Migration / MigrationContext / MigrationReport /
  AppliedMigration / MIGRATIONS with Codemod / CodemodContext /
  CodemodReport / AppliedCodemod / CODEMODS.
- Codemod.apply signature changes from `fn(&mut Mapping, &Ctx) ->
  Result<()>` to `fn(&mut Mapping, &Ctx) -> Result<bool>` so each
  codemod reports whether it actually modified the mapping.
- Simplify the runner: flat per-codemod loop, no version arithmetic,
  no contiguity checks, no overflow guards, no future-version
  rejection.
- Drop hoist_schema_version from reconstruct_source — no
  schema-version field to hoist.
- Update parse_markdown_detailed, compile_pipeline_inner,
  perform_source_rewrite_if_needed, check_pipeline, run_execute,
  and compile_all_pipelines to use the codemod types and reword
  their messages.
- Helpers (take_key, insert_no_overwrite, rename_key,
  ConflictPolicy) are unchanged — they're as useful for codemods as
  they were for migrations. rename_key already returned
  Result<bool>, matching the new codemod contract.
- Drop ~13 schema-version-specific tests (registry contiguity,
  future version, non-integer, etc.). Add 7 codemod-runner tests
  (empty registry, all-no-op, mixed, error context, idempotency).
  Black-box CLI tests reduced from 10 to 3 (deleted tests asserted
  on schema-version-specific behavior that no longer exists).

PR #476 has not merged; this lands as a follow-up commit on the
same branch so users never see the schema-version concept ship.

All 1349 tests pass. PR title and body to be updated separately.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jamesadevine

AGENTS.md

@@ -61,10 +61,10 @@ Every compiled pipeline runs as three sequential jobs:
 │   │   │   ├── safe_outputs.rs # Always-on SafeOutputs MCP extension
 │   │   │   ├── trigger_filters.rs # Trigger filter extension (gate evaluator delivery)
 │   │   │   └── tests.rs  # Extension integration tests
-│   │   ├── migrations/   # Front-matter migrations (one file per version step)
-│   │   │   ├── mod.rs    # Migration struct, MIGRATIONS registry, runner, CURRENT_SCHEMA_VERSION
+│   │   ├── codemods/     # Front-matter codemods (one file per transformation)
+│   │   │   ├── mod.rs    # Codemod struct, CODEMODS registry, runner
 │   │   │   └── helpers.rs # take_key, insert_no_overwrite, rename_key, ConflictPolicy
-│   │   ├── migration_integration_test.rs # White-box rewrite-path tests (stub registry injection)
+│   │   ├── codemod_integration_test.rs # White-box rewrite-path tests (stub registry injection)
 │   │   └── types.rs      # Front matter grammar and types
 │   ├── init.rs           # Repository initialization for AI-first authoring
 │   ├── execute.rs        # Stage 3 safe output execution
@@ -196,9 +196,10 @@ index to jump to the right page.
 - [`docs/filter-ir.md`](docs/filter-ir.md) — filter expression IR
   specification: `Fact`/`Predicate` types, three-pass compilation (lower →
   validate → codegen), gate step generation, adding new filter types.
-- [`docs/migrations.md`](docs/migrations.md) — front-matter migration
-  framework: schema-version field, automatic source rewrite on
-  breaking-change updates, contributor workflow for adding migrations.
+- [`docs/codemods.md`](docs/codemods.md) — front-matter codemod
+  framework: detection-based transformations, automatic source
+  rewrite on breaking-change updates, contributor workflow for
+  adding codemods.
 - [`docs/local-development.md`](docs/local-development.md) — local development
   setup notes.