Collapse full-ci flake fixes for verification

[starr-openai]

Why

Verify whether the current candidate full-ci flake fixes go green when applied together on a fresh origin/main branch, without changing the source PRs.

What

Collapse these fix PRs into one side branch for a single rust-ci-full verification run:

• Emit unified exec end on startup failure #22743
• Stabilize remote full-ci test startup paths #22742
• Fix Windows sandbox helper discovery in Cargo tests #22741
• Keep plugin test home alive during MCP startup #22910
• Split slow exec resume cwd test #22922
• Stabilize non-git cwd test fixtures #23447
• test: deflake realtime sideband close #21506
• Stabilize guardian shell timeout test #23528
• Stabilize ambient git test fixtures #23597
• Hide deferred tools from code mode prompt #23598
• Keep network denial tests off remote exec path #23599
• Stabilize standalone user shell context test #23614
• Stabilize compact remote parity skills normalization #23621
• Preserve code mode output history budget #23622

#23447 and #23597 carry patch-identical non-git cwd fixture fixes, so this verification branch includes that fix once.

Verification

• Intended validation: fresh rust-ci-full run on this side branch.
• Local validation not run; the source PRs already carry their focused validation and this branch exists only to answer the combined full-ci question.