LSP Phase 8: feature gate + Channels struct refactor

[yogthos]

Phase 8 of 9. Stacked on #31 (Phase 7).

Two related changes in one PR. Splitting them would have required disentangling overlapping diffs in the same files (both touch the LSP-related fields in tool structs / call sites / Channels).

Channels struct refactor

Phase 7's `build_channels` returned a 10-tuple that grew every time we added a slot. Now it returns a named struct:

```rust
#[derive(Default)]
struct Channels {
permission: Option,
ask_tx: Option,
ask_rx: Option,
question_tx: Option,
question_rx: Option,
plan_tx: Option,
plan_rx: Option,
bg_store: Option,
lifecycle_rx: Option,
#[cfg(feature = "lsp")]
lsp_manager: Option<std::sync::Arc>,
}
```

main.rs destructures the same locals it already used. No behavior change, but the call site is no longer a positional 10-tuple unpacking.

Also doc-noted: `compile_lsp_commands` currently ignores the `extensions` field on per-server overrides — the claimed-extensions list lives in the static `builtin_servers()` registry. Follow-up.

Phase 8: `feature = "lsp"` (default-on)

Added `lsp` to the default feature set. With it off (`cargo build --no-default-features --features 'loop git-worktree mcp'`):

• `lsp` module not compiled; `lsp-types` dep stays optional + skipped.
• `Channels.lsp_manager` field gated out (also gated out of destructure in main.rs).
• `build_agent` / `build_agent_inner` / `run_interactive` drop their `lsp_manager` arg via `#[cfg(feature = "lsp")]` on the param.
• Read/Write/Edit tools drop their `lsp_manager` field + integration call.
• `LspTool` not registered; `--no-lsp` CLI flag not exposed.
• `LspConfig` / `LspServerConfig` types gated; `cfg.lsp` field gated.
• ACP path, slash sub-rebuilds, plan-switch all use `#[cfg(feature = "lsp")]` on the `None` args.

Verified configurations

Command	Result
`cargo build --no-default-features --features 'loop git-worktree mcp'`	Clean, no lsp deps
`cargo build` (default)	Clean, all 4 LSP servers wired
`cargo test --bin dirge --no-default-features --features 'loop git-worktree mcp' -- --skip plugin`	305 passing
`cargo test --bin dirge -- --skip plugin` (default)	426 passing

The +121 tests with the feature on are the LSP module's tests + the diagnostic integration tests, correctly excluded when the feature is off.

Test plan

• Both feature configurations build clean
• Both feature configurations test clean
• `cargo fmt --check` clean

Next: Phase 9 (docs + manual end-to-end test against real rust-analyzer).