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/core/error.rs

@@ -53,7 +53,7 @@ impl ErrorCode {
     pub fn exit_code(&self) -> i32 {
         match self {
             Self::InternalError => 1,
-            Self::ConfigNotFound => 2,
+            Self::ConfigNotFound => 20,
             Self::ConfigInvalid => 3,
             Self::TokenNotSet => 4,
             Self::GitLabAuthFailed => 5,