LSP Phase 6: edit-tool diagnostic integration

[yogthos]

Phase 6 of 9. Stacked on #29 (Phase 5).

After every `write` / `edit`, the LSP server's fresh diagnostics surface in the tool result so the agent corrects errors on the same turn — instead of writing broken code and discovering it via a manual `cargo check` round-trip.

What's in this PR

`src/lsp/diagnostic.rs` (new)

Function	Purpose
`pretty(d)`	One-line `SEVERITY [line:col] msg` with 0→1-based coordinate conversion
`report(file, issues)`	Wraps in `<diagnostics file="...">` block. ERROR severity only. Caps at 20 with `... and N more` overflow.
`build_report_block(current, all)`	The full agent-facing block: current-file errors + up to 5 other-file errors

15 diagnostic tests covering severity labels, coordinate conversion, error filtering, MAX_PER_FILE cap with overflow footer, other-files cap, warning-only-files-don't-appear, and caller-path preservation through canonicalize lookup.

`src/agent/tools/write.rs` + `edit.rs`

New optional `Option<Arc>` field. When set, after the write/edit:

1. `manager.touch_file(path, TouchMode::AwaitPush { after: write_at, timeout: 10s })` so the server processes `didChange` and publishes fresh diagnostics.
2. `manager.all_diagnostics()` → `diagnostic::build_report_block` → append to tool output.

The shared `append_lsp_block` helper lives in `write.rs` and is called from both tools. Errors during touch/wait are swallowed — diagnostic surfacing is a side-effect; the primary tool contract is "wrote/edited the file."

`src/agent/tools/read.rs`

New optional `Option<Arc>` field. Read fires off a `tokio::spawn(touch_file(Notify))` so the server has the file open by the time the agent edits it. No diagnostic block in the read's output — that's the edit tool's job.

Test coverage

• Diagnostic module: 15 tests (severities, error-only filter, caps, path preservation regression).
• WriteTool: 2 integration tests — no-manager path preserves pre-LSP output exactly (regression); manager-with-no-diagnostics appends nothing.
• Pre-LSP tests for write/edit/read still pass (no behavior change in the no-manager path).

Code review fixes applied before push

• Display the caller's path (not the canonical form) in the current-file diagnostic section so macOS `/tmp ↔ /private/tmp` asymmetry doesn't show the agent a path it didn't type. Regression test added.

Scope

• builder.rs passes `None` for all three tool constructors' new `lsp_manager` arg. Phase 7's CLI/config plumbing will create the manager from build_channels and populate this.
• Tools' integration is end-to-end-functional once the manager is wired through; the no-manager path is exercised by tests, the with-manager path is exercised by the diagnostic-block-formatting tests + a manual test in Phase 9.

Test plan

• `cargo build` clean
• `cargo test --bin dirge -- --skip plugin` → 422 passing (was 405; +17 from Phase 6)
• `cargo fmt --check` clean
• End-to-end manual test against rust-analyzer: edit a file with a deliberate compile error, observe the diagnostic block in the edit tool's output — comes in Phase 9.

Next: Phase 7 (config + CLI plumbing — `build_channels` creates the `LspManager` with `ProcessSpawner` configured for the 4 built-in servers; threads through every `build_agent` call site).