fix(security): close urgent trust-boundary gaps

[Panniantong]

Scope

Addresses #378, #508, #514, and #523.

• make install read-only by default and require explicit --system authorization for host writes
• propagate installer failures and support resolved Windows command shims
• keep credentials out of output and documented process arguments
• require explicit consent before transcription crosses providers
• bound and validate Xiaoyuzhou download, media, and transcription inputs
• harden transcription URLs against shorthand and ambiguous internal-host spellings
• constrain the V2EX TLS fallback to the exact recoverable failure boundary
• refresh LinkedIn to the current uvx stdio contract
• use only the Exa tools actually registered by the live MCP
• replace stale integration commands with a clean isolated end-to-end test

Verification

• pytest tests/ -q: 528 passed
• clean bash test.sh run: 528 passed
• Ruff: passed
• Mypy on changed source modules: passed
• shell syntax and git diff checks: passed
• live V2EX, Exa, LinkedIn, and version-check smokes: passed

Coordination

• preserves original contributor attribution from PRs fix(v2ex): recover from Python TLS EOF #536, fix(skill): use registered Exa search tools #537, and fix(transcribe): block shorthand IPv4 spellings of internal hosts #564
• deliberately excludes fix: decode yt-dlp and ffmpeg output as UTF-8 #549 and the Web, OpenCode, and localized retired-channel scope owned by fix: harden web reads and complete P1 cleanup #576
• ready for review; do not merge until the planned fix: harden web reads and complete P1 cleanup #576 overlap review is complete