feat(cli): add --config flag to override config.ini location (#164)

[nicolasbisurgi]

Summary

Adds an optional --config PATH flag (long-form only) to every TM1-connecting command — run, build, tasks …, resume — letting a single invocation point at an arbitrary config.ini (TM1 connection parameters). The driving use case: sharing one read-only config.ini with other tm1py utilities (e.g. OptimusPy) instead of duplicating credentials.

Closes #164.

Design (per ADR 0003)

• Resolution precedence (highest wins): --config > RUSHTI_DIR > legacy CWD > config/.
• Explicit threading, no global mutation. The resolved path flows as config_path into the TM1 connection layer (connect_to_tm1_instance, setup_tm1_services). The module-level CONFIG global in cli.py is demoted to the no-flag default (still computed at import for its legacy-path deprecation-warning side effect). The from rushti.cli import CONFIG fallback in execution.py is removed in favour of a required, explicit argument.
• Relocates only config.ini — settings.ini keeps --settings, logging_config.ini keeps its own resolution.
• Fail fast on a bad path: exit 1, clean message, no traceback.
• Not added to stats/db (no TM1 connection).
• No behavioural change when the flag is absent — resolution is byte-for-byte today's behaviour.

Changes

Source

• app_paths.py — shared add_config_arg(parser) helper (mirrors add_log_level_arg).
• cli.py — flag on the run parser; config threaded into both cli_args dicts; resolve + fail-fast after parsing; fed into all three connect sites.
• execution.py — removed the CONFIG import fallback; config_path now required.
• commands/build.py, commands/resume.py (forwarded into the rebuilt argv), commands/tasks/__init__.py (all 5 subparsers + wired the existing stub).

Tests — tests/unit/test_config_flag.py (16 cases): helper parsing, run threading + fail-fast, build resolution + bad-path, all 5 tasks subcommands forwarding config_path, and resume surviving the argv rebuild.

Docs — README, CHANGELOG, CONTEXT.md, docs/advanced/cli-reference.md, docs/advanced/settings-reference.md.

Testing

• Full unit suite: 788 passed, 5 skipped.
• ruff + black clean (enforced by pre-commit).
• Manual: verified --config appears on run/build/tasks/resume help, is absent on stats/db, -c still maps to resume --checkpoint, and a missing path exits 1 with a clean message.

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