feat: override merge mode for generate-context

[joshbouncesecurity]

Summary

When generate-context detects a manual override file (OPENANT.md / OPENANT.json), it now prompts the user to choose:

• use — apply the override as-is, skip the LLM call.
• merge — feed the override into the LLM alongside other sources.
• ignore — skip the override, generate from scratch.

Adds --override-mode <use|merge|ignore> to bypass the prompt for CI/automation. --force is kept as a backward-compatible shortcut for --override-mode ignore. In non-interactive environments (no TTY) the prompt is skipped and use is the default.

Previously, override files were all-or-nothing — either they fully replaced LLM generation or were ignored entirely, with no way to combine developer-provided hints with LLM analysis.

Depends on #41 (item 17 — generate-context CLI command). Please merge that first; the diff here is the override-mode addition only.

Addresses item 18 from #16 (does not close the issue).

Test plan

• Interactive prompt offers use/merge/ignore when an override exists.
• --override-mode use skips the LLM and emits the override verbatim.
• --override-mode merge feeds the override into the LLM (verify with --show-prompt).
• --override-mode ignore ignores the override.
• --force still works as a shortcut for --override-mode ignore.
• --force and --override-mode are mutually exclusive (CLI rejects both).
• No-TTY default is use.

[joshbouncesecurity]

Manual verification

Requires API key. Drop an OPENANT.md (or OPENANT.json) in the target repo first.

• openant generate-context <repo> (interactive TTY): prompts to choose use / merge / ignore.
• openant generate-context <repo> --override-mode use: skips the LLM call entirely; emits the OPENANT.md content as the application context.
• openant generate-context <repo> --override-mode merge --show-prompt: prompt sent to the LLM includes the override file content + the merge supplement.
• openant generate-context <repo> --override-mode ignore: ignores the override; generates from scratch.
• openant generate-context <repo> --force: behaves identically to --override-mode ignore.
• openant generate-context <repo> --force --override-mode merge: rejected with a clear mutual-exclusion error.
• No-TTY: openant generate-context <repo> < /dev/null (or piped through CI): skips the prompt, defaults to use.
• Priority: with both OPENANT.md and OPENANT.json present, the .md wins.
• Note: depends on PR feat: add generate-context CLI command with auto-discovery #41; please merge that first.

[joshbouncesecurity]

Local test results

Built and reinstalled openant-core from this branch. Created a tiny fixture (app.py + OPENANT.md declaring application_type: cli_tool) and exercised the new flag.

Commands run:

go build -o openant.exe ./
pip install -e libs/openant-core
openant --json generate-context <fixture> --override-mode merge --show-prompt
openant generate-context <fixture> --force --override-mode merge       # mutex check

Outcome (per checklist):

• --override-mode merge --show-prompt: API call ran with Sonnet (~6.6s), Source: merged, generated context returned application_type: cli_tool (matching the override), confidence: 100% ✅
• The JSON-mode output's prompt_format field is populated and clearly contains the OPENANT.md override content (Prints hardcoded 'hi' string on invocation, Hardcoded 'hi' return value - this is the intended functionality, etc.) ✅
• --force --override-mode merge: rejected with Error: --force and --override-mode are mutually exclusive ✅
• generate-context --help lists --override-mode and explains the three modes ✅
• Did not run the use / ignore / interactive-prompt / no-TTY / .md-vs-.json priority paths in this manual pass — those are covered by tests in the diff.

Note for the maintainer (small UX nit): --show-prompt only surfaces the formatted prompt when the global --json flag is also set (since the prompt goes through _output_json). The prompt-format text never appears in the friendly stdout output, so a user typing openant generate-context --show-prompt without --json sees no visible prompt. Worth either documenting that pairing or always emitting the prompt to stdout when the flag is passed.

Reported cost: $0.00 (Sonnet, ~6.9s).