v0.15.2 — reconciliation + Swift ABI grammar + grammar_updater pin verification

Reconciliation release. The release/v0.15.x line and main had drifted apart; v0.15.2 unifies them so subsequent work has a single forward branch to build on.

Added

• Swift ABI mangling grammar (grammargen/swift_abi_grammar.go, SwiftABIManglingGrammar()). Parses the $s / $S / $e / _T0 Swift symbol-mangling prefixes. Intended for tooling that needs to walk demangled Swift symbols without invoking the Swift toolchain.
• cmd/grammar_updater -verify-pins flag. Validates that every locked commit in grammars/languages.manifest is still fetchable from its declared remote before any sync runs. verifyRemotePins / verifyRemoteCommit deduplicate by repo+commit to keep the check cheap on large manifests.
• cmd/grammar_updater -sync-manifest-only flag. Limits a sync pass to manifest entries that are new since the last run. syncMissingEntriesFromManifest now returns a map so callers can apply an allow-list filter.

Changed

• Plan-doc directories are now gitignored (.claude/, docs/blog-outlines/, docs/plans/, docs/superpowers/) along with the benchgate binary. Plan docs are working references and should not ship with the repo.

Removed

• Four stray plan-doc files that had been committed under docs/plans/ and docs/superpowers/ prior to the gitignore update.