feat: ai — gemini terse+render+last+session+machine, openrouter new tool, capture any command

gemini enhancements:
- built-in terse system prompt with troubleshooting clause + machine context
- markdown→terminal rendering (glow opportunistic + zero-dep awk fallback)
- --last: pos logs + captured output fallback, staleness warning, stderr annotations
- session default always on; --session override; answer separation on tty
- --full flag, --system wholesale override

openrouter (new tool):
- cloned from gemini, adapted for OpenAI-compatible REST API
- Bearer auth, messages array, choices[0].message.content parsing
- config: pos config ai-openrouter → OPENROUTER_API_KEY/MODEL
- default model: openrouter/auto (auto-picks best model)
- all features: ask, chat, sessions, --last, capture

capture subcommand (both tools):
- runs any command, tees output to last_cmd_output for --last
- --last fallback: pos logs (priority) → last_cmd_output (secondary)

shell hook (optional):
- lib/pos-ai-hook.sh: sourceable .bashrc snippet for auto-capture
- exec > >(tee ...) with 1 MB truncation

lib/pos-ai-hook.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Optional shell hook for pos ai * --last: auto-captures terminal output.
+# Usage: add to ~/.bashrc:
+#   source /usr/local/bin/pos-ai-hook.sh
+#   — or —
+#   source /path/to/Linux_post_install/lib/pos-ai-hook.sh
+#
+# After sourcing, every command's stdout+stderr is silently tee'd to
+# ~/.local/share/linux_post_install/last_cmd_output (truncated at 1 MB).
+# Then pos ai gemini ask --last / pos ai openrouter ask --last will
+# pick it up automatically — no 'capture' subcommand needed.
+# To disable: unset __POS_CAPTURE_ACTIVE
+
+__POS_CAPTURE_FILE="${HOME}/.local/share/linux_post_install/last_cmd_output"
+__POS_CAPTURE_MAX=${__POS_CAPTURE_MAX:-1048576}  # 1 MB, override with env
+
+# Truncate if oversized (keep last half)
+if [ -f "$__POS_CAPTURE_FILE" ]; then
+    __sz=$(stat -c%s "$__POS_CAPTURE_FILE" 2>/dev/null || echo 0)
+    if [ "$__sz" -gt "$__POS_CAPTURE_MAX" ]; then
+        tail -c $((__POS_CAPTURE_MAX / 2)) "$__POS_CAPTURE_FILE" > "${__POS_CAPTURE_FILE}.tmp" 2>/dev/null
+        mv -- "${__POS_CAPTURE_FILE}.tmp" "$__POS_CAPTURE_FILE"
+    fi
+else
+    : > "$__POS_CAPTURE_FILE"
+fi
+
+# Only activate in interactive terminals, not already redirected
+if [ -t 1 ] && [ -t 2 ] && [ -z "${__POS_CAPTURE_ACTIVE:-}" ]; then
+    export __POS_CAPTURE_ACTIVE=1
+    exec > >(tee -a "$__POS_CAPTURE_FILE" 2>&1) 2>&1
+fi