docs: clarify auto_fixable in JSON actions[] is per-finding

[BartWaardenburg]

Summary

The JSON actions[] array has two semantically distinct shapes of auto_fixable: per-rule (the rule type can be auto-fixed; constant across every finding for that issue type) and per-instance (the bool depends on the specific finding's payload, not the action type). The distinction matters for agents that filter on auto_fixable: true to decide what is safe to apply blindly.

Document per-finding evaluation across every surface. No behavior change.

• Enum-level doc on IssueAction and per-field docs on FixAction.auto_fixable and AddToConfigAction.auto_fixable in crates/types/src/output.rs call out the rule and enumerate the four current per-instance flips.
• New field_definitions: { actions[], actions[].auto_fixable } block in the check / health / dupes JSON _meta payloads carries the same wording (shared via two const strings in crates/cli/src/explain.rs so wording stays in sync).
• docs/output-schema.json regenerated via cargo run -p fallow-cli --features schema-emit --bin fallow-schema-emit: three description fields updated on IssueAction, FixAction.auto_fixable, AddToConfigAction.auto_fixable.
• VS Code + npm typed contracts (output-contract.d.ts) regenerated via pnpm run codegen:types so JSDoc reflects the new wording.
• Two new unit tests in crates/cli/src/explain.rs assert the per-finding wording and the shared constants across all three _meta builders.
• CHANGELOG entry under [Unreleased] -> Documentation.
• Companion repos: matching updates in fallow-rs/docs (analysis/auto-fix.mdx adds a "per-finding semantics" section; cli/dead-code.mdx tightens the actions[] table row) and fallow-rs/fallow-skills (cli-reference.md table row + per-instance flip list).

The four currently active per-instance flips, all documented in the new prose:

• remove-catalog-entry (unused-catalog-entries): true only when the finding's hardcoded_consumers array is empty (else fallow fix skips the entry to avoid breaking pnpm install).
• remove-dependency vs move-dependency (dependency findings): primary action flips on used_in_workspaces.
• add-to-config for ignoreExports (duplicate-exports): true only when a fallow config file exists on disk.
• update-catalog-reference (unresolved-catalog-references): always false today; non-singleton on the wire so a future applier can promote it without a schema change.

All suppress-line and suppress-file actions remain uniformly false.

Closes #361.

Test plan

• cargo test --workspace --all-targets (37 test results, 0 failures)
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo fmt --all -- --check clean
• cargo doc --workspace --no-deps --document-private-items no warnings
• pnpm run check:codegen clean after regen (both output-contract.d.ts files regenerated and committed)
• cargo test -p fallow-cli --lib check_meta_documents_per_finding_auto_fixable passes (asserts the wording calls out PER FINDING, remove-catalog-entry, used_in_workspaces, ignoreExports)
• Manual spot-check: cargo run -p fallow-cli --features schema-emit --bin fallow-schema-emit | diff docs/output-schema.json - shows only the three intended description updates
• json-output-reviewer agent pass (APPROVE; one minor non-blocking note about omitting add-catalog-entry from the always-false bucket, intentionally out of scope: this PR documents per-instance flips, not every always-false action type)

[BartWaardenburg]

Released in v2.76.0.