feat: bash-backed glob/grep discovery tools (dsh-tool-fs-search)

Implements docs/rfc/implemented/feature/2026-07-09-bash-backed-grep-glob-
discovery.md: model-facing glob/grep in a new @deepseek-ai/dsh-tool-fs-search
package, executing fixed ripgrep templates through ctx.bash.resolve/run —
not ctx.fs provider methods — so filesystem backends stay free of a search
contract and sandboxed/remote executors substitute cleanly. The tools never
call ctx.bash.start(); the tool layer owns quoting (one singleQuote safety
boundary), rg --json parsing, ItemRetainer/TextRetainer retention, and the
first tool-owned ctx.spillFiles.saveText() handoff (item-level retention the
generic post-execute spill policy cannot recover).

RFC amendments on the way to implemented/: a shared src/search-core.ts (the
SEARCH_* vocabulary + bash-run/raw-spill/spill plumbing was byte-identical
across both tools — the missed-extraction smell), and a snapshot-gap note:
wiring the acp-agent tree changes the assembled prompt, so goldens need a
keyed re-record; the spill notice text is pinned by unit tests instead and
only the coding-agent example ships the tools for now.

examples/coding-agent/composition.md

@@ -43,6 +43,8 @@ flowchart LR
   cfg --> plugin_coding_fs_policy
   plugin_coding_tool_fs["tool-fs<br/>@deepseek-ai/dsh-tool-fs"]
   cfg --> plugin_coding_tool_fs
+  plugin_coding_tool_fs_search["tool-fs-search<br/>@deepseek-ai/dsh-tool-fs-search"]
+  cfg --> plugin_coding_tool_fs_search
   plugin_coding_spill_local["spill-local<br/>@deepseek-ai/dsh-spill-local"]
   cfg --> plugin_coding_spill_local
   plugin_coding_spill_policy["spill-policy<br/>@deepseek-ai/dsh-spill-policy"]
@@ -65,6 +67,7 @@ flowchart LR
 | `fs-local` | `@deepseek-ai/dsh-fs-local` |
 | `fs-policy` | `@deepseek-ai/dsh-fs-policy` |
 | `tool-fs` | `@deepseek-ai/dsh-tool-fs` |
+| `tool-fs-search` | `@deepseek-ai/dsh-tool-fs-search` |
 | `spill-local` | `@deepseek-ai/dsh-spill-local` |
 | `spill-policy` | `@deepseek-ai/dsh-spill-policy` |