codegraff TUI: feature parity with graff REPL + rendering polish

[justrach]

Why

The codegraff TUI (crates/codegraff-tui/) is the future home for richer agent UX (subagent observability, image previews, multi-pane workflows — see #15). But it currently lags the graff REPL on the basics: only a subset of slash commands, no autocomplete, occasional rendering artifacts when overlays open/close, no clipboard image paste, no parity with REPL features users rely on.

Without closing this gap, the TUI feels like a second-class experience and users default to graff.

What

This is a tracking issue. Each item below is a candidate sub-issue or PR.

Feature parity (port from graff REPL)

• Slash command palette + autocomplete (covered in companion issue)
• /model switching with the same model picker UX as graff
• /provider management
• /conversation browse / resume / rename / delete
• /agent switching (forge, muse, sage)
• /info, /usage, /dump, /compact, /help
• Image paste (clipboard → multimodal — companion issue covers graff side)

Rendering polish (artifact hunt)

• Audit the Frame::render path in crates/codegraff-tui/src/main.rs for places that draw without Clear first
• Trace overlay close paths — confirm full-screen redraw happens on close, no ghost regions
• Verify resize handling — repaint everything on Resize events, not just the dirty parts
• Check the transcript scroll math when multiline messages truncate
• Check that the workflow-trace pane doesn't double-print after re-rendering
• Consider extracting main.rs (210 KB single file) into a tui/ module tree — single file makes future work harder

Out of scope for this issue (separate trackers)

• [Feature]: Add visual TUI observability for subagents #15 (subagent visual observability) — different scope, large
• Image rendering inline (terminal protocol work) — fights line-based regions, deferred

Acceptance

This issue stays open until at least 80% of the parity checklist lands and an artifact-free run can be demonstrated on a recorded session. Sub-issues should reference back here.

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