LSP Phase 5: agent-facing lsp tool

src/agent/tools/lsp.rs (LspTool)
- One tool, one `operation` parameter. Operations mirror opencode's
  surface so agents' mental model carries over:
    definition / references / hover / documentSymbol / workspaceSymbol /
    implementation / prepareCallHierarchy / incomingCalls / outgoingCalls
- Accepts both camelCase (goToDefinition) and short (definition) aliases.
- 1-based line/character (matches editor display); converts to 0-based
  before sending to the LSP wire. saturating_sub guards against agents
  passing 0.
- Relative file_path resolved against the dirge worktree cwd.
- Permission check via check_perm_path on the resolved absolute path.
- touch_file(TouchMode::Notify) before each request so the file is in
  sync with the server. Notify (not AwaitPush) — diagnostics are the
  edit tool's concern in Phase 6.
- Empty result reported as '(no results from <op>)' rather than '[]'
  so the agent doesn't have to special-case.

14 tool tests including 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; silent empty result)
- workspaceSymbol requires query
- missing file on disk returns clean error
- 1-based -> 0-based coordinate conversion (regression for off-by-one
  that 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)

Builder.rs wiring (Option<Arc<LspManager>> threaded through, tool
attached when manager is present) lands in Phase 7 with the rest of the
CLI/config plumbing. Phase 5's tool symbols are #![allow(dead_code)]
until then to keep the warning surface clean.

Phase 4: 86, Phase 5: +14 -> 100 LSP tests. Suite: 391 -> 405.

Author: Yogthos