✨ Add 'xhigh' effort level and default to it (refs #81)

[monkut]

Summary

• Adds XHIGH = "xhigh" to VALID_EFFORT_LEVELS between HIGH and MAX (Opus 4.7-only effort tier from Anthropic's effort docs).
• Changes DEFAULT_EFFORT_LEVEL from MAX to XHIGH to match Anthropic's recommended starting point for coding/agentic work on Opus 4.7.
• Bumps version to 0.2.5 (pyproject.toml + uv.lock).

Refs #81

Notes

• askcc/cli.py uses choices=VALID_EFFORT_LEVELS and the help string renders settings.ASKCC_CLAUDE_EFFORT_LEVEL dynamically, so --effort xhigh and the new default surface automatically.
• _resolve_effort_level() iterates the enum for its warning message — no code change there, but I confirmed ASKCC_CLAUDE_EFFORT_LEVEL=xhigh no longer triggers the invalid-value warning.
• README does not reference effort levels, so no doc updates needed there.

Test plan

• uv run poe check (ruff) — passes
• uv run poe typecheck (pyright) — 0 errors
• uv run poe test — 155 passed (incl. 2 new tests: test_xhigh_effort_level_resolves_without_warning, test_default_effort_level_is_xhigh)
• Existing tests for low/high/max/invalid values still pass unchanged