docs: fix agent doc links and capture extension test gotchas

[BenWeekes]

What

• Fix AGENTS.md entry-point links: L2 deep dives live at docs/ai/L1/L2/ on main (the file pointed at a nonexistent docs/ai/L1/deep_dives/, so agents following the entry point missed all eight deep dives); also note that progressive-disclosure-standard.md lives on the docs/progressive-disclosure branch, not main.
• Extension development deep dive: add ASR guarder contract gotchas learned from recent ASR extension work — positive/non-overlapping final timestamps (synthesize from the audio timeline when the vendor has no timing), invalid credentials must be non-fatal with reconnect-manager escalation (only HTTP 401/403 immediately fatal), connection_status tests are allowlist-gated, and capture guarder output to a log file to keep the pytest summary/exit code. Add the newer same-session finalize-reconnect and connection-status tests to the guarder table.
• Setup: fresh-worktree bootstrap ordering (task install first; install_python_deps.sh alone fails before ten_packages/system exists).
• Workflows: in-container lint always exits non-zero with E0401 import errors; judge against a known CI-green extension baseline.

Why

Process docs only — no code changes. These are the exact points that cost debugging time in recent extension work; capturing them makes the next extension land right the first time.