Fix security, reliability, and quality issues from verification sweep

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Summary

Full verification-and-fix pass across the top issues, module hotspots, and quick wins:

Security

• Command/shell injection: execSync string interpolation replaced with argv-array execFileSync / node:fs in postinstall-patch.mjs, native-addons.ts, and the download/clean-up scripts; Windows shell: true seam in shell.ts hardened.
• Webview XSS: escapeHtml now escapes quotes, inline onclick attributes removed, CSP tightened (no unsafe-inline, explicit connect-src), remote Google Fonts @import removed.
• Cross-window message handling: all four webview listeners validate event.data via a shared guard (webview/messages.ts) before trusting payloads; message-serializer parses defensively.
• Path traversal: @mention resolution constrained to the workspace root; MOCHA_TEST_FILE and loader.cjs sub-path imports refuse root escape.

Reliability / correctness

• Fail-open CI fixed: signal-terminated and spawn-failed runs reported distinctly (verify.mjs, fallow-audit.mjs, run-node-tests.mjs, runTest.ts, test scaffold); watchdog/no-result paths exit non-zero.
• pi-binary.ts bare-name resolution works again (was spawning a shell builtin with shell: false).
• loader.cjs: package-name typo fixed, doc URLs extracted, timeouts on which/where, no duplicate process-wide Module._resolveFilename hook, bare imports resolved via the package exports map.
• Host-side: per-item settle on multi-session delete, voice temp-file/listener cleanup, Mermaid cache cap + stale-render race fix, PiPackagesPanel batched registry lookups with stale-response discard, App.svelte clears stale tool state on errors/new session.
• Hardcoded values removed: machine-specific home path, Linux distro paths, fixed tmpdir singletons, pinned VS Code version → env/config.

Quality

• Mocks no longer mask SDK drift (overrideable Proxy, disposeCalls tracking, no double listener fire, no dup handler registration).
• Deduplicated PiAgentConfig/ThinkingLevel/SessionNode and the per-component sendMessage clones; consolidated ABI-scan logic.
• a11y wiring (aria-describedby, Space handling, focusable dialogs), prefers-reduced-motion guards, timer/cache/log caps.
• Config hygiene: invalid pnpm-workspace.yaml keys removed, tsconfig redundancy trimmed, .vscodeignore no longer ships nested env/secret files.

Verification

• pnpm run verify: all 6 stages green (check, lint, format, 661 unit tests, webview tests, fallow audit — 0 introduced findings).

Changelog

Updated CHANGELOG.md under [Unreleased] with a line per fix.

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Co-Authored-By: Codebuff noreply@codebuff.com