feat(app): single-source the G1 capture dump onto the workspace snapshot (F-mcp-snapshot-g1, #216)

[bastien-gallay]

What

The G1 dev-loop dump (~/.termherd/captures/capture-<ts>.json) and the MCP
snapshot tool described the same application with two models:
CaptureDump (tabs + the focused terminal) and WorkspaceSnapshot (config +
sidebar + per-pane tabs + scoped terminals). Two models means a field can mean
one thing on disk and another on the wire, and every later rung has to teach
both. This collapses them onto one.

before                                  after
Event::Capture{focused_pty_text}        Event::Capture(SnapshotInputs)
        │                                       │
App::build_capture ──► CaptureDump      App::snapshot(&SnapshotFilter::capture(), …)
        │                                       │
Effect::Capture(CaptureDump)            Effect::Capture(WorkspaceSnapshot)
        │                                       │
capture::DumpDto ──► JSON               snapshot_dto::SnapshotDto ──► JSON
                                        (the same one the MCP tool uses)

SnapshotFilter::capture() is the fixed shape behind a capture: every
structural section, plus the focused pane's screen kept whole. A capture is
written to a file, not streamed into a call, so it pays for the full picture —
but it stays scoped to the pane the developer is looking at when they press the
key.

core::capture (CaptureDump / CaptureTab) is gone, and the shell gathers a
capture's adapter inputs through the same seam a bridge snapshot uses
(snapshot_inputs_for), so config and terminal text are collected exactly one
way.

Two commits

1. refactor(app) — tidy-first, no behaviour change: the JSON wire form of
   WorkspaceSnapshot moves out of mcp/handler.rs into app::snapshot_dto,
   field-for-field, so both readers can share it.
2. feat(app) — the reframe above.

Breaking dump-format change

Deliberate, and the point of the slice:

• focused_pty: "<text>" → terminals: { "<handle>": "<text>" }
• the dump gains focus, config, sidebar, and per-pane detail (stable
  handle, kind, cwd, status)
• status words follow the MCP vocabulary — "ready" becomes "idle" — which
  retires strings::status_label (it existed only for this dump)

The contract an AI actually depends on — newest capture = highest-named
pair — is untouched. AGENTS.md's capture note is refreshed.

Tests

Written red first, then made green. 8 tests over the new behaviour:

• core::snapshot — the capture filter is every section + the focused terminal,
  untruncated (10 000 lines survive).
• core::app::capture — the dump carries every section; the focused text rides
  whole; a background pane's injected text is not included; a tab's custom
  title wins; empty workspace; split pane order (the last two are regressions
  kept from the old suite).
• app::capture — the on-disk JSON is the MCP shape (string handles, "idle").
• app::shell — end-to-end through the perform_capture dir seam: the written
  file holds focus, tabs, config and the focused terminal.

Full suite green (cargo test --workspace); fmt · clippy · machete ·
deny · check-deps · check-arch · markdownlint all pass. ROADMAP entry
ticked.

Closes #216.