feat(mcp): show running sync progress in sync_status

[kyle-elliott]

Part 4 of the #37 stack: #42 → #43 → #44 → this PR. It is a follow-up to #44.

Problem

After #44, index_codebase returns immediately and agents poll sync_status. During the first index: syncing phase, sync_status did not say how far the scan or upload had come. The background reconcile used the quiet sync::sync, so its progress went nowhere.

Change

• The coordinator owns a progress slot (src/engine/coordinator/progress.rs, a tokio::sync::watch sender). ReconcileRun carries it, and SyncReconciler calls sync_with_progress and publishes each milestone. A drop guard around the reconcile clears the slot on success, failure, panic, and cancellation.
• CoordinatorStatus.sync_progress: Option<SyncProgress> (#[serde(default)], so a status body from an older daemon still decodes). SyncProgress itself is the wire type (snake_case tags); no second shape exists.
• One renderer: impl Display for SyncProgress. semctl index, the sync_status line, and semctl daemon status use it. The upload text changes from uploading: 3/8 files to uploading 3/8 files, so the status line does not read current sync: uploading: ….
• sync_status prints current sync: <milestone> after the first index: line while a reconcile runs, for example current sync: uploading 1200/3607 files. The line is absent when nothing runs.
• CoordinatorStatus and its Display move to src/engine/coordinator/status.rs, because coordinator.rs was at the 1,000-line limit (983 → 956).
• The quiet sync::sync wrapper had no caller left, so it is removed.

Verification

• Full gate passes (fmt, test, clippy, doc, git diff --check).
• Tests:
  • The coordinator shows Uploading { 3, 10 } while a fake reconcile runs, and the slot is empty after success, failure, and cancellation.
  • Old-daemon status decoding and a round-trip with progress.
  • The renderer text for each milestone.
  • The position of the current sync: line in sync_status, and that it is absent when idle.
• Live smoke test: I started an MCP server in ~/napbat/symtree with an isolated runtime directory and polled sync_status. It showed current sync: scanned 1294 files (0 filter decisions reused) — checking for changes during the startup reconcile, then no line after the reconcile ended. The checkout had no changed files, so the live run did not reach the upload step. Unit tests cover the upload counts.