fix(cli): fail cleanly when a DB-requiring command has no backend

[syf2211]

Summary

Add a shared guard in TjCommand.invoke that raises a ClickException when a DB-requiring leaf command runs with ctx.obj["db"] is None, instead of crashing with an AttributeError traceback.

Motivation

Issue #727: commands that skip open_db() (including the --help skip path from #726) but still execute can reach DB-touching code with db=None. cmd_optimize already had a hand-rolled message; other commands like loop annotate did not.

Changes

• Add db_required_message() and _ensure_db_available() to TjCommand (leaf commands only; groups skip the guard)
• Set ctx.obj["requires_db"] in cli/main.py (False for no_db_commands, True otherwise)
• Remove the redundant db is None check from cmd_optimize; reuse the shared message for the ApiBackend path
• Add unit + integration regression tests

Tests

pytest tests/unit/test_tj_db_guard.py tests/integration/test_cli.py::test_db_requiring_command_fails_cleanly_when_db_is_none tests/integration/test_cli.py::test_nested_subcommand_help_skips_db_probe -v
ruff check tokenjam/cli/tj_status.py tokenjam/cli/main.py tokenjam/cli/cmd_optimize.py
mypy tokenjam/cli/tj_status.py tokenjam/cli/main.py

All passed locally.

Notes

• Builds on the nested --help DB skip from fix(cli): skip DB probe when nested subcommand requests --help #726 (fix/group-subcommand-help-no-db). If fix(cli): skip DB probe when nested subcommand requests --help #726 merges first, this branch can be rebased to a single commit on top of main.
• Related follow-up: CLI: --help detection should skip tokens Click consumes as an option value #728 (ignore --help tokens consumed as option values) is separate.

Fixes #727

[greptile-apps]

Greptile Summary

The PR adds a centralized Click command guard that converts missing-backend crashes into actionable errors for database-requiring leaf commands.

• Propagates database requirements through the root CLI context.
• Reuses a shared missing-database message in the optimize command.
• Adds unit and integration coverage for missing backends and nested help.

Confidence Score: 5/5

The PR appears safe to merge, with no actionable defects identified in the centralized database guard or its command coverage.

The current registered database-requiring leaves route through TjCommand, database-independent top-level commands retain their opt-out behavior, and missing backends now fail through an actionable Click exception.

Important Files Changed

Filename	Overview
tokenjam/cli/tj_status.py	Adds shared database-requirement metadata, missing-backend validation, and help-token detection to the central command class.
tokenjam/cli/main.py	Records whether the selected top-level command requires a database and skips database opening on the nested-help path.
tokenjam/cli/cmd_optimize.py	Delegates missing-backend handling to the shared guard and reuses the centralized error message.
tests/unit/test_tj_db_guard.py	Verifies shared error formatting, leaf-command enforcement, and group-command exemption.
tests/integration/test_cli.py	Covers nested help without a database probe and clean failure when a database-requiring leaf executes without a backend.

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