feat(tui): phase labels for non-bootstrap phases (LLM call, schema diff, review lanes)

Deferred from the v0.9.3 pre-release review.

### Current behavior

`packages/tui/src/util/phase-label.ts` maps five `bootstrap.*` phase names to human labels ("Loading session...", "Discovering tools...", etc.). Once bootstrap completes, every subsequent phase name falls through to "Thinking..." — the same default the idle spinner uses.

### Gap

For long-running dbt review runs (schema diffing, LLM lane, tool listing), the label stays on "Thinking..." for the whole run — no signal that the tool is actively doing work versus stalled. This reduces the value of the bootstrap phase-label UX added in this release.

### Proposal

Extend the phase-label map with per-workload labels (e.g. `review.diff`, `review.equivalence`, `review.ai-lane`) once the latency-instrumentation from the bootstrap trace spans provides enough signal to pick meaningful groupings. Start with the review pipeline since it has the highest wall-clock and the loudest complaint surface.

### Why deferred

The bootstrap instrumentation just landed. Picking non-bootstrap phase groupings without the latency data risks labels that don't match the actual work — a follow-up when we can point at real spans.

Start with packages/tui/src/util/phase-label.ts to understand the existing bootstrap mappings and fallback behavior. Then inspect the review pipeline and the latency-instrumentation trace spans to identify meaningful non-bootstrap groupings. Done means the selected review phases have accurate labels grounded in observed spans and the labels no longer use the generic idle wording during those workloads.

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