feat(cli): complete enumerated flag values and model IDs

[sergiught]

📝 Summary

Flags with a closed set of values fell back to filename completion, so pressing Tab offered filesystem paths where only a handful of words are valid:

# before
$ ofga __complete query check --consistency ""
:0                      # ShellCompDirectiveDefault — i.e. complete filenames

# after
HIGHER_CONSISTENCY
MINIMIZE_LATENCY
UNSPECIFIED

Covered: --consistency, --file-format, --on-duplicate, --on-missing, the global --theme, profiles add --auth-method, and the <key> argument of profiles set/profiles unset — which also completes auth_method's values once the key is typed.

model graph and assertions read/assertions test take an optional [model-id] but offered nothing for it, unlike model get. All three now use the same completion.

Two small consolidations rather than new copies: --consistency completion is registered by the shared helper that defines the flag (so any future command offering it gets this for free), and the model-ID completion moves to internal/cli instead of being written a third time.

🏷 Type of change

• 🐛 Bug fix (non-breaking)
• ✨ Feature (non-breaking)
• 💥 Breaking change (users must update)
• ♻️ Refactor (no functional change)
• 📚 Docs only
• 🛠 Build / CI / tooling

✅ How to verify

make check

Or drive the protocol a shell uses:

ofga __complete query check --consistency ""
ofga __complete profiles set auth_method ""

TestEnumeratedFlagsCompleteTheirValues runs each case through cobra's __complete; every one returned only the :0 filename directive before this change.

Note: touches internal/command/profiles/command.go, so it will need a trivial rebase if #109 (profile headers) merges first.

📋 Checklist

• 🧪 Tests added or updated where it makes sense
• 📖 Docs updated (README / command --help / examples) if behavior changed — completion only, no help text changes
• 🏷 PR title follows Conventional Commits
• 🟢 make check passes locally