feat(fix): auto-apply ignoreExports rules to fallow config (closes #330)

[BartWaardenburg]

Summary

fallow fix can now apply the add-to-config action for duplicate-export findings directly to the user's fallow config file, instead of just emitting a paste-ready snippet. The action flips from auto_fixable: false to auto_fixable: true whenever a config file exists; with no config, fix skips with a clear Run 'fallow init' to create one message.

This unblocks shadcn / Radix / bits-ui-style namespace-barrel projects where users had to hand-paste 30+ ignoreExports entries.

What ships

• Writer module fallow_config::add_ignore_exports_rule covers all four supported config formats atomically. JSON/JSONC go through jsonc-parser (format-preserving CST); TOML / .fallow.toml go through toml_edit. Both round-trip comments, ordering, inline-vs-newline shape, trailing commas, CRLF/LF line endings, and a UTF-8 BOM.
• Merge semantics: append-only, dedupe on exact file match, no reorder, no replace. Absolute existing entries dedupe against the writer's relative emissions when they resolve under the config dir.
• Workspace path re-anchoring: when the config lives in a workspace subdir (e.g. packages/ui/.fallowrc.json), emitted paths are relative to that config's directory.
• Symlink-safe writes: atomic_write canonicalizes the target before persisting so configs mounted as symlinks (common in Docker setups) get their target updated rather than being replaced with a regular file.
• Parser convergence: all json_comments::StripComments parse sites replaced with jsonc-parser so reads and writes share the same JSONC dialect.
• Schema: AddToConfigAction.auto_fixable widens from const false to boolean. TypeScript contract regenerated for both editors/vscode/src/generated/output-contract.d.ts and npm/fallow/types/output-contract.d.ts.

Test plan

• cargo test -p fallow-cli -p fallow-config --all-targets
• cargo clippy -p fallow-cli -p fallow-config -- -D warnings
• cargo fmt --all -- --check
• typos
• check:codegen (regenerated TS contract is in sync)
• End-to-end smoke combining BOM-prefixed JSON config + symlinked path + pre-existing absolute-path entry: write succeeds, BOM preserved (count=1), symlink intact, no abs/rel duplicate, post-fix dead-code reports 0 duplicates, second fix is byte-idempotent
• TOML BOM round-trip end-to-end smoke
• Workspace-subdir re-anchor smoke (packages/ui/.fallowrc.json emits src/index.ts not packages/ui/src/index.ts)
• Missing-config skip message verified

Out of scope (separate issues)

• Optional value_schema URL field on add-to-config actions for AI-agent input validation
• Missing-config-file auto-creation policy and fallow fix --dry-run config diff preview
• Threading explicit --config <path> through into auto_fixable (narrow edge case: user passes --config pointing outside root's ancestors AND agent gates on auto_fixable rather than attempting the fix)