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>

README.md

@@ -406,8 +406,8 @@ 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 |
@@ -419,6 +419,19 @@ 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
+
+Settings are resolved in this order (highest to lowest priority):
+
+1. CLI flags (`--robot`, `--config`, `--color`)
+2. Environment variables (`LORE_ROBOT`, `GITLAB_TOKEN`, `LORE_CONFIG_PATH`)
+3. Config file (`~/.config/lore/config.json`)
+4. Built-in defaults
 
 ## Global Options
 
@@ -428,6 +441,24 @@ lore --robot <command>                    # Machine-readable JSON
 lore -J <command>                         # JSON shorthand
 ```
 
+## Shell Completions
+
+Generate shell completions for tab-completion support:
+
+```bash
+# Bash (add to ~/.bashrc)
+lore completions bash > ~/.local/share/bash-completion/completions/lore
+
+# Zsh (add to ~/.zshrc: fpath=(~/.zfunc $fpath))
+lore completions zsh > ~/.zfunc/_lore
+
+# Fish
+lore completions fish > ~/.config/fish/completions/lore.fish
+
+# PowerShell (add to $PROFILE)
+lore completions powershell >> $PROFILE
+```
+
 ## Database Schema
 
 Data is stored in SQLite with WAL mode and foreign keys enabled. Main tables: