feat(coding-agent): integrate Markdown transformer API

[CoderDoubleflower]

Integrate upstream Pi PR earendil-works#7231 as an isolated feature while preserving pi-claude's Claude-style assistant marker, compact transcript spacing, prompt-height user background, task status UI, branding, release identity, TodoWrite, compaction features, and npm runtime compatibility.

The integration adds a display-only, chainable Markdown transformation API with message type, streaming state, and available terminal width context. It does not merge the rest of the pending upstream commit range.

Validation target: build, check, full tests, and packed npm installation smoke test.

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[CoderDoubleflower]

Integration validation

Integrated upstream Pi PR earendil-works#7231 as an isolated feature. The automatic three-way apply was clean for every changed file except the fork-specific assistant and user message components; those two were manually composed to preserve the Claude-style ●/❯ markers, compact spacing, prompt-height background, thinking presentation, and OSC 133 zones.

Validation completed successfully in the integration job:

• monorepo build
• repository check/type validation
• assistant and user message transformer tests
• extension discovery and runner tests
• TUI Markdown tests
• packed npm installation smoke test

The upstream width assertions were adapted from 78/58 to 77/57 because the fork's visible message marker and configured padding consume one additional terminal column. This reflects the exact content width available to transformed Markdown.

No TodoWrite, compaction, branding, release identity, tool rendering, task-status, or npm runtime compatibility files were replaced.