test: achieve 97% coverage and remove avatar feature (#10)

* test: achieve 97% coverage and remove avatar feature

- Add comprehensive tests across all packages to raise coverage from
  75.9% to 97.4% (remaining 2.6% is structurally unreachable dead code)
- Add codecov.yml to exclude auto-generated cmd/generated/ and main.go
  from coverage reporting (18K lines dragging Codecov to 17.95%)
- Remove internal/avatar package and cf avatar command (unused feature)
- Remove avatar docs and sidebar entry

Package coverage: audit 100%, cache 100%, config 100%, diff 100%,
duration 100%, errors 100%, jq 100%, jsonutil 100%, policy 100%,
gen 100%, client 95.9%, oauth2 97.7%, template 98.9%, gendocs 97.3%,
cmd 79.0%

* chore: remove .planning/ from git tracking

Already in .gitignore but was still tracked. Files remain on disk.

* fix: prevent test from opening browser

TestOpenBrowserCurrentOS was calling the real openBrowser() which
runs `open https://example.com` on macOS, launching the user's browser.
Mock the function instead.

* fix: resolve errcheck lint violations in test files

Author: sofq