refactor: make --full/-f a command-level flag instead of global (#877)

* refactor: make --full/-f a command-level flag instead of global

Move --full/-f from global flags (parsed in flags.rs) to command-level
parsing in commands.rs, scoped to the three commands that actually use
it: `screenshot`, `diff screenshot`, and `diff url`.

This frees up `-f` for other commands (e.g. `--follow` on
`console`/`errors`, see #867) and better reflects that full-page
capture is not a global concern.

Changes:
- Remove `full` from Flags struct, Config struct, and global flag parsing
- Remove `--full`/`-f` from clean_args global boolean flags list
- Parse `--full`/`-f` inline in `screenshot` command handler
- Accept `-f` shorthand in `diff screenshot` and `diff url` (previously
  only `--full` was accepted at command level)
- Remove fallback from global `flags.full` in diff subcommands
- Update tests to pass --full as a command argument rather than a global flag

Fixes #876

* fix: remove stale AGENT_BROWSER_FULL env var from help and add -f shorthand tests

- Remove AGENT_BROWSER_FULL from help text in output.rs since the env
  var is no longer read after moving --full to command-level parsing
- Add test_screenshot_full_page_shorthand to verify screenshot -f works
- Add test_diff_screenshot_command_full_flag_shorthand to verify
  diff screenshot -f works

---------

Co-authored-by: ctate <366502+ctate@users.noreply.github.com>

cli/src/output.rs

@@ -2655,7 +2655,6 @@ Environment:
   AGENT_BROWSER_EXTENSIONS       Comma-separated browser extension paths
   AGENT_BROWSER_HEADED           Show browser window (not headless)
   AGENT_BROWSER_JSON             JSON output
-  AGENT_BROWSER_FULL             Full page screenshot
   AGENT_BROWSER_ANNOTATE         Annotated screenshot with numbered labels and legend
   AGENT_BROWSER_DEBUG            Debug output
   AGENT_BROWSER_IGNORE_HTTPS_ERRORS Ignore HTTPS certificate errors