docs: Update exit codes, add config precedence and shell completions

Exit code tables (README + AGENTS.md):
- Add codes 14-16 (Ollama unavailable, model not found, embedding failed)
- Add code 20 (Config not found, remapped from 2)
- Clarify code 1 (now includes health check failed + not implemented)
- Clarify code 2 (now exclusively usage/parsing errors from clap)

New sections:
- Configuration Precedence: CLI flags > env vars > config file > defaults
- Shell Completions: bash, zsh, fish, powershell installation instructions

Co-Authored-By: Claude (us.anthropic.claude-opus-4-5-20251101-v1:0) <noreply@anthropic.com>

AGENTS.md

@@ -98,8 +98,8 @@ Errors return structured JSON to stderr:
 | Code | Meaning |
 |------|---------|
 | 0 | Success |
-| 1 | Internal error |
-| 2 | Config not found |
+| 1 | Internal error / health check failed / not implemented |
+| 2 | Usage error (invalid flags or arguments) |
 | 3 | Config invalid |
 | 4 | Token not set |
 | 5 | GitLab auth failed |
@@ -111,6 +111,17 @@ Errors return structured JSON to stderr:
 | 11 | Migration failed |
 | 12 | I/O error |
 | 13 | Transform error |
+| 14 | Ollama unavailable |
+| 15 | Ollama model not found |
+| 16 | Embedding failed |
+| 20 | Config not found |
+
+### Configuration Precedence
+
+1. CLI flags (highest priority)
+2. Environment variables (`LORE_ROBOT`, `GITLAB_TOKEN`, `LORE_CONFIG_PATH`)
+3. Config file (`~/.config/lore/config.json`)
+4. Built-in defaults (lowest priority)
 
 ### Best Practices