feat(tui): spill completed results for every tool

[hartsock]

What this PR does

Implements round 2 of #1235: every completed tool result now uses one operator-facing header plus the configured tail-biased spill view. This covers built-ins, routed shell commands, delegated tools, synthetic repeat-suppression/context results, prompt/artifact-safe projections, reports, and cancellation.

The dispatch boundary preserves full model results while showing raw shell tails to the operator. It adds regression coverage for canonical audit labels, privacy, errors, no output, report rendering, and pre-start cancellation.

Test plan

• cargo build --workspace
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• just check
• env LLVM_COV=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/llvm-cov LLVM_PROFDATA=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/llvm-profdata just cov-ci (87.50%, floor 80%)

Out of scope

This is completion-time spill presentation, not interactive terminal scrollback. Arrow/thumb scrolling and live tail-follow require the separate streaming/repaint architecture and a superseding plain-scroller decision.