LSP Phase 5: agent-facing lsp tool

[yogthos]

Phase 5 of 9. Stacked on #28 (Phase 4).

The agent-facing tool that dispatches to the Phase 4 `LspManager`. Mirrors opencode's `tool/lsp.ts` surface so agents' mental model carries over.

What's in this PR

`src/agent/tools/lsp.rs` (LspTool)

One tool, one `operation` parameter. Operations:

• `definition` / `references` / `hover`
• `documentSymbol` / `workspaceSymbol`
• `implementation` / `prepareCallHierarchy` / `incomingCalls` / `outgoingCalls`

Accepts both camelCase (`goToDefinition`) and short (`definition`) aliases.

Coordinates: 1-based line/character (matches editor display); converts to 0-based before sending to the LSP wire. `saturating_sub` guards against agents passing `0`.

Paths: relative `file_path` resolved against the dirge worktree cwd. Permission check via `check_perm_path` on the resolved absolute path.

File sync: `touch_file(TouchMode::Notify)` before each request so the file's in sync with the server. `Notify` (not `AwaitPush`) — diagnostics are the edit tool's concern in Phase 6.

Empty results: reported as `(no results from )` rather than `[]` so the agent doesn't have to special-case.

Test coverage (14 tests, includes regressions)

• Unknown operation returns clear error (regression — bad name shouldn't panic or produce a cryptic parse error)
• Position op without `file_path` errors (regression — would crash a `manager.hover(None, ...)` otherwise)
• Position op without line/character errors
• `workspaceSymbol` requires `file_path` (regression — without one, no LSP attaches and the request silently returns nothing)
• `workspaceSymbol` requires `query`
• Missing file on disk returns clean error
• 1-based → 0-based coordinate conversion (regression — off-by-one would land the LSP cursor on the wrong identifier)
• `documentSymbol` accepts no position (regression — `needs_position` must not include doc/workspace symbol)
• Successful hover returns pretty-printed JSON
• Empty array / null response reports `(no results)`
• Accepts opencode camelCase aliases
• Relative `file_path` resolves against cwd (regression)
• `line=0` / `character=0` doesn't panic (saturating_sub guards)

Code review fixes applied before push

• `workspaceSymbol` silent-failure mode: tool initially marked `file_path` optional, but the manager picks which LSP server to ask based on the file's extension. With no `file_path` (cwd has no extension) no server matches and the request returns silently. Fixed: `file_path` is now required for every operation (matches opencode). Added regression test.
• `Operation::from_str`: renamed to `parse` to avoid shadowing `std::str::FromStr`.

Scope

This PR lands the tool + tests. The `builder.rs` wiring that actually attaches the tool to a running agent comes in Phase 7 with the rest of the CLI/config plumbing (Option<Arc> threading through `build_channels` / `build_agent` / slash rebuilds, ProcessSpawner construction with default server commands). The Phase 5 tool symbols carry `#![allow(dead_code)]` until then.

Test plan

• `cargo build` clean
• `cargo test --bin dirge -- --skip plugin` → 405 passing (was 391; +14 from Phase 5)
• `cargo fmt --check` clean
• End-to-end against real rust-analyzer comes in Phase 6+9 once the orchestrator is wired into the agent.

Next: Phase 6 (write/edit tool diagnostic integration — surface LSP errors in tool output via `TouchMode::AwaitPush`).