Add 'xhigh' to VALID_EFFORT_LEVELS (Opus 4.7 effort value)

[ellen-goc]

Summary

askcc/settings.py defines VALID_EFFORT_LEVELS as {low, medium, high, max}, which is missing the xhigh level introduced with Claude Opus 4.7. Setting ASKCC_CLAUDE_EFFORT_LEVEL=xhigh currently triggers the invalid-value warning and falls back to the default.

Anthropic's authoritative list (docs):

Value	Availability
low	all supported models
medium	all supported models
high	all supported models (API default)
xhigh	Opus 4.7 only — missing from askcc-cli
max	Mythos Preview, Opus 4.7, Opus 4.6, Sonnet 4.6

Per the docs, xhigh is the recommended starting point for coding and agentic work on Opus 4.7, so askcc-cli should both support it and default to it.

Proposed Approach

In askcc/settings.py:

1. Add XHIGH = "xhigh" to the VALID_EFFORT_LEVELS enum between HIGH and MAX.
2. Change DEFAULT_EFFORT_LEVEL from VALID_EFFORT_LEVELS.MAX to VALID_EFFORT_LEVELS.XHIGH — aligns askcc-cli's default with Anthropic's recommended starting point for coding/agentic work on Opus 4.7 and avoids the "overthinking" / high-cost profile of max on structured tasks.
3. Verify _resolve_effort_level() warning message still renders the full list correctly (it iterates the enum, so no code change needed there).
4. Update any docs/README references to the supported effort levels and the new default.

Acceptance Criteria

• VALID_EFFORT_LEVELS enum includes XHIGH = "xhigh".
• DEFAULT_EFFORT_LEVEL resolves to VALID_EFFORT_LEVELS.XHIGH when ASKCC_CLAUDE_EFFORT_LEVEL is unset.
• ASKCC_CLAUDE_EFFORT_LEVEL=xhigh resolves without the "Invalid ASKCC_CLAUDE_EFFORT_LEVEL" warning.
• Unit test covers the new value and the new default.
• README / env-var documentation lists xhigh as a valid value and reflects the new default.

References

• Anthropic effort parameter docs: https://platform.claude.com/docs/en/build-with-claude/effort
• Comment-add commit that surfaced the enum: 41fe723

[monkut]

Confirmed satisfied by #82 (merged). Verified against current main:

• askcc/settings.py:43 — XHIGH = "xhigh" added between HIGH and MAX.
• askcc/settings.py:47 — DEFAULT_EFFORT_LEVEL = VALID_EFFORT_LEVELS.XHIGH.
• _resolve_effort_level() iterates the enum, so xhigh no longer triggers the invalid-value warning.
• tests/test_askcc.py:1586 — test_xhigh_effort_level_resolves_without_warning.
• tests/test_askcc.py:1595 — test_default_effort_level_is_xhigh.
• README has no effort-level references, so no doc update was needed.

Closing.