Code navigation: symbol index and jump to definition #293

repo grep and repo search find text; there is no way to go from a name to its definition.

• Index definitions per commit on the default branch (and optionally MR heads), built by a background worker after push.
• Indexer choice decides the cost: universal-ctags is a new runtime dependency on bay1 and the runner image; a Go-side tokenizer limits the language set.
• repo symbols <owner/name> [--ref r] <query> lists matches with path and line.
• Web: identifiers in the blob view link to their definition when the index has exactly one match, and to a result list otherwise. A symbol list per file.
• Index keyed by tree so an unchanged tree is not reindexed (as CI dedupe does).