deps: remove jq — route gist-envelope + address parsing through airc_core

[joelteply]

Why

airc requires both jq and python3. Since #152's truth-layer migration we already have python3 as a hard dep, so jq is functionally redundant — it parses JSON, which python's stdlib does natively. Joel called this out 2026-04-28: "why do we have so many damn dependencies now?"

Current jq usage (28 occurrences in airc, plus install.sh prereq + doctor probe)

Two clusters:

• Address parsing (airc:777-824) — host_addresses array filtering by .scope == "localhost" / "lan" / "tailscale", returning addr|port pairs.
• Gist envelope parsing (airc:1892-1988) — extracting .invite, .name, .kind, .airc, .host.addresses, .host.machine_id, .last_heartbeat.

Both are JSON reads with no in-place mutation. Trivial to route through a new airc_core.envelope (or airc_core.json) CLI that takes JSON via stdin / --content and emits the requested field on stdout.

Outcome

• Drop jq from install.sh / install.ps1 / doctor probes
• Drop the jq-related fallback paths in airc (each reader currently has command -v jq guards that fall through silently — those become unnecessary)
• One fewer thing for users to install on a clean machine, one fewer thing to break on the Windows install path

Out of scope

This is bash-only — airc.ps1 already uses native PowerShell JSON parsing (ConvertFrom-Json).