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>

build.rs

@@ -7,4 +7,5 @@ fn main() {
         .unwrap_or_default();
     println!("cargo:rustc-env=GIT_HASH={}", hash.trim());
     println!("cargo:rerun-if-changed=.git/HEAD");
+    println!("cargo:rerun-if-changed=.git/refs/heads");
 }