fix: shutdown safety, CLI hardening, exit code collision

Shutdown signal improvements:
- Upgrade ShutdownSignal from Relaxed to Release/Acquire ordering.
  Relaxed was technically sufficient for a single flag but
  Release/Acquire is the textbook correct pattern and ensures
  visibility guarantees across threads without relying on x86 TSO.
- Add double Ctrl+C support to all three signal handlers (ingest,
  embed, sync). First Ctrl+C sets cooperative flag with user message;
  second Ctrl+C force-exits with code 130 (standard SIGINT convention).

CLI hardening:
- LORE_ROBOT env var now checks for truthy values (!empty, !="0",
  !="false") instead of mere existence. Setting LORE_ROBOT=0 or
  LORE_ROBOT=false no longer activates robot mode.
- Replace unreachable!() in color mode match with defensive warning
  and fallback to auto. Clap validates the values but defense in depth
  prevents panics if the value_parser is ever changed.
- Replace unreachable!() in completions shell match with proper error
  return for unsupported shells.

Exit code collision fix:
- ConfigNotFound was mapped to exit code 2 (error.rs:56) which
  collided with handle_clap_error() also using exit code 2 for parse
  errors. Agents calling lore --robot could not distinguish "bad
  arguments" from "missing config file."
- Restore ConfigNotFound to exit code 20 (its original dedicated code).
- Update robot-docs exit code table: code 2 = "Usage error", code 20 =
  "Config not found".

Build script:
- Track .git/refs/heads directory for Cargo rebuild triggers. Ensures
  GIT_HASH env var updates when branch refs change, not just HEAD.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/mod.rs

@@ -76,7 +76,9 @@ impl Cli {
         let args: Vec<String> = std::env::args().collect();
         args.iter()
             .any(|a| a == "--robot" || a == "-J" || a == "--json")
-            || std::env::var("LORE_ROBOT").is_ok()
+            || std::env::var("LORE_ROBOT")
+                .ok()
+                .is_some_and(|v| !v.is_empty() && v != "0" && v != "false")
             || !std::io::stdout().is_terminal()
     }
 }