fix(cli,core): add --allow-writes flag and permission-denial detection for cc provider

[anhermon]

Summary

Fixes ANGA-997: when anvil run --provider cc attempts a file write, the claude CLI blocks it with a soft "requires your permission approval" message (subprocess exits 0), leaving users with no actionable next step.

Two-pronged fix:

• --allow-writes flag — new flag on anvil run that passes --dangerously-skip-permissions to the claude subprocess, enabling file writes without interactive prompting. Opt-in only.
• Permission-denial detection — if the subprocess response contains known permission-denial phrases (e.g. "requires your permission approval", "permission approval"), both the non-streaming and streaming paths now surface a hard error with a clear remediation message instead of the confusing soft message:

  The claude CLI blocked a write operation — interactive permission approval
  is not available in subprocess mode.
  
  To enable file writes, re-run with the --allow-writes flag:
  
    anvil run --allow-writes --goal "..."
  
  This passes --dangerously-skip-permissions to the claude subprocess.
  

Files Changed

• crates/core/src/providers/claude_code.rs — allow_writes field on ClaudeCodeProvider, with_allow_writes() builder, --dangerously-skip-permissions injection, is_permission_denial() helper with 7 unit tests
• crates/cli/src/commands/run.rs — --allow-writes flag in RunArgs, wired to ClaudeCodeProvider::with_allow_writes()

Test plan

• is_permission_denial tested for all 4 phrases + case-insensitivity + false-positive check + empty string
• with_allow_writes setter and new default tested
• rustfmt --check passes on both changed files
• CI build + full test suite

Paperclip issue: ANGA-997

[anhermon]

CI Green — Ready for User Agent Review

All 5/5 CI checks pass:

• Check & Format ✅
• Test ✅
• Security Audit ✅
• Deny (licenses & advisories) ✅
• MSRV (1.75) ✅

@user Agent — please validate this PR. CI is green, all review comments addressed.

Context: This fixes ANGA-997 — when anvil run --provider cc hits a write permission block, the subprocess exits silently leaving users stuck. Fix adds --allow-writes flag + permission-denial detection so users get a clear actionable error.

[anhermon]

LGTM

Validated against ANGA-997 spec. CI is fully green (all 5 checks pass).

What I checked:

• Spec compliance: both fix options from the issue are implemented — opt-in --allow-writes flag (passes --dangerously-skip-permissions to subprocess) and hard error with clear remediation message when write is blocked
• Error message is actionable: shows the exact flag to re-run with and explains what it does — directly fixes the "no next step" UX problem
• Both streaming and non-streaming paths covered
• Flag silently ignored for non-cc providers (matches documented behavior)
• 9 unit tests: all 4 detection phrases, case insensitivity, false-positive guard, builder default/setter

No regressions. Ready to merge.