apps/cli: build WP sites incrementally

[dereksmart]

Related issues

See p1776889791701059/1776881604.011089-slack-C08LZ79MKG9

The turn immediately after site_create (and after any "redesign / rebuild /
start over" prompt) was consistently producing one huge assistant response —
planning essay + multiple Writes + full theme scaffold all in one turn. At
Anthropic streaming rates that's 10–15 minutes of model generation. The CLI
TUI only renders complete assistant messages (no stream_event handler), so
the whole window is a silent "Meandering…" spinner; upstream gateways
sometimes time out with 504 before the response finishes.

How AI was used in this PR

Proposed Changes

• Add a "Working cadence" section to the local system prompt: one Write/Edit per turn, skeleton-first for long files (>~200 lines), short prose between tool calls, and a hard cap on the turn that immediately follows any site-wide scaffolding trigger.
• Frame cadence as a quality lever (screenshot loop) alongside UX, so the model doesn't read it as a cap on ambition.
• Expand the site-spec skill's "After site_create returns" section with matching rules at the single biggest offending point.

Testing Instructions

• studio ai → "make me a site" → runs site-spec, creates site, next turn is small (site_info or short Write), not a 15-minute silent span
• studio ai with existing site → "redesign the site" → cadence rule for scaffolding triggers kicks in; no mega-turn
• Visual quality on the final build is on-par with or better than before (screenshots mid-build are exercised)
• Existing test suites (apps/cli/ai/tests/*) still green — tests mock buildSystemPrompt, so content changes don't affect them

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 4100410 vs trunk

app-size

Metric	trunk	4100410	Diff	Change
App Size (Mac)	1491.75 MB	1491.75 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	4100410	Diff	Change
load	1602 ms	1908 ms	+306 ms	🔴 19.1%

site-startup

Metric	trunk	4100410	Diff	Change
siteCreation	8107 ms	8128 ms	+21 ms	⚪ 0.0%
siteStartup	4956 ms	4950 ms	6 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[dereksmart]

done in #3199