feat: add Thin v6 atop context-efficient ACP sessions

[reinhold-ph]

Summary

Apply the accepted Thin-v6 desktop product patch on top of the isolated context-efficient upstream baseline in #9.

Buzz v6 remains a lean desktop client: it discovers and verifies compatible external Codex and Claude runtimes rather than bundling large CLI/ACP installations. The baseline supplies block/buzz#5423, which delivers standing ACP context once per live session and sends only newly delivered thread/DM event deltas on later turns.

Buzz thread
  -> buzz-acp live channel session
     -> standing context once
     -> only newly delivered event IDs on later turns
  -> verified external Codex/Claude runtime plan
  -> isolated Buzz v6 test application

Review scope

• Base: sync/upstream-context-efficiency at 85b87cbf… (chore: sync upstream through context-efficient ACP sessions #9)
• Head: 9009dae3…
• 1 commit
• 23 changed files
• 1,483 additions / 329 deletions
• No .github/workflows changes

The patch carries the accepted Thin-v6 behavior from source 3d273ec15ef7d4d3f8a33638603b0bc6800f5dbf, with the integration adaptations needed on the newer upstream baseline:

• verified external runtime discovery and compatibility planning;
• fail-closed launcher identity;
• candidate-isolated state and keyring behavior;
• runtime provenance surfaced in the UI;
• Rust 1.95 formatting;
• managed_agents/runtime.rs kept within the 1000-line source-size ratchet.

The obsolete thin-v6-macos-arm64.yml workflow and later deliberately failing test commit a30377a2… are not included.

Dependency and gates

• Depends on chore: sync upstream through context-efficient ACP sessions #9; this PR must not merge first.
• Does not dispatch a build, publish a release, install an app, or merge itself.
• Build/package, native product acceptance, publication, and release/signing remain separate approval gates.
• Restart continuity remains separate and unresolved (block/buzz#5342).
• Negotiated developer-instruction transport from block/buzz#5386 / agentclientprotocol/codex-acp#379 is not added here.

Verification

Passed on the exact head before this ref-only split:

• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --all -- --check
• cargo fmt --manifest-path Cargo.toml --all -- --check
• node desktop/scripts/check-file-sizes.mjs
• node desktop/scripts/check-px-text.mjs
• node desktop/scripts/check-pubkey-truncation.mjs
• Targeted Biome check for all changed frontend files
• git diff --check
• Staged secret-pattern scan
• Independent exact-tree review: PASS, no blockers

Reverified during the split:

• git diff --check 85b87cbf..9009dae3 passes.
• Exactly 23 paths differ.
• No workflow path differs.
• Both local commits retain Signed-off-by trailers.

Not run on the combined head:

• cargo test -p buzz-acp remains blocked offline by uncached anyhow 1.0.104.
• Tauri runtime tests remain blocked offline by uncached portable-pty.
• Full frontend typecheck/tests remain blocked because the fresh worktree has no lockfile-matching node_modules; older worktree dependencies were intentionally not reused.
• No macOS build or native acceptance was run on this combined head.

The earlier Thin-v6 candidate passed native macOS acceptance separately, and upstream block#5423 passed its own CI, but the final merged result still requires the normal post-merge exact-SHA build and acceptance gate.