feat!: Migrate to Arcanist-compatible configuration

[holmboe]

Summary

• Read .arcconfig from git repo root for PHAB_URL (via phabricator.uri)
• Setup wizard (phabfive user setup) now writes credentials to ~/.arcrc in Arcanist-compatible JSON format instead of ~/.config/phabfive.yaml
• Deprecate PHAB_URL/PHAB_TOKEN in ~/.config/phabfive.yaml with DeprecationWarning guiding users to migrate
• When PHAB_URL is missing, offer to create .arcconfig instead of running the full ~/.arcrc setup wizard
• Env-var PHAB_URL is peeked at before .arcrc loading so multi-host ~/.arcrc works with env overrides

Backwards incompatibility

This is not a breaking change in the strict sense — existing configurations continue to work. However it is backwards incompatible in these ways:

• phabfive user setup now writes to ~/.arcrc (JSON) instead of ~/.config/phabfive.yaml (YAML). Users who re-run setup will get credentials in the new location.
• Users with PHAB_URL/PHAB_TOKEN in ~/.config/phabfive.yaml will see deprecation warnings on stderr.
• The error message for missing PHAB_URL now suggests creating .arcconfig instead of editing phabfive.yaml.

New config precedence

1. Hard-coded defaults
2. /etc/phabfive.yaml
3. /etc/phabfive.d/*.yaml
4. ~/.config/phabfive.yaml         (PHAB_URL/PHAB_TOKEN deprecated, warns)
5. ~/.config/phabfive.d/*.yaml     (same)
6. .arcconfig in git root          (provides PHAB_URL)      [NEW]
7. ~/.arcrc                        (provides PHAB_TOKEN)
8. Environment variables           (override everything)

Test plan

• uv run pytest — 330 tests pass
• uv run ruff check — clean
• Manual: PHAB_URL=... phabfive user whoami works in empty directory with multi-host ~/.arcrc
• Manual: phabfive user setup creates/updates ~/.arcrc in correct JSON format
• Manual: Place .arcconfig in a git repo root, verify phabfive reads phabricator.uri
• Manual: Put PHAB_URL in ~/.config/phabfive.yaml, verify deprecation warning appears

Closes #171, closes #172, closes #173

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