Support user OpenCode profiles under ~/.config/agentic/opencode/profiles and show them in the profile menu

[sawrus]

Motivation

• Allow users to add local OpenCode model profiles that can be selected during interactive installs without modifying bundled extensions.
• Surface user profiles in the same optional plugin/profile menu as built-in profiles so installs can apply them seamlessly.
• Keep profile application logic unified so bundled and user profiles are handled the same when writing .opencode/opencode.json.

Description

• Added OPENCODE_USER_PROFILES_DIR and new helper functions opencode_builtin_profile_contains, opencode_user_profile_ids, and opencode_profile_source_path to discover and prefer bundled profiles when present.
• Renamed and adjusted opencode_profile_* utilities so labels for user profiles are displayed as <profile-id> profile and opencode_profile_id_from_label strips that suffix for lookup.
• Updated interactive plugin/profile selection to include user profiles from $HOME/.config/agentic/opencode/profiles/<profile-id>/opencode.json and to set SELECTED_OPENCODE_PROFILE for chosen user profiles.
• Modified profile application to use opencode_profile_source_path so installs copy from bundled profiles when available or from the user profile path otherwise.
• Documentation updates in README.md, docs/opencode_setup.md, and docs/agentic-usage.md documenting the user profile directory and examples.
• Extended tests/e2e/opencode_plugins.e2e.sh to create sample user profiles (DT and GH) and add assertions that the menu lists and applies those profiles correctly.

Testing

• Ran the updated end-to-end test tests/e2e/opencode_plugins.e2e.sh which creates DT and GH profiles under a fake HOME and verifies menu entries and applied profiles; the test passed.
• Existing assertions in the same e2e test that check built-in profiles and final project files were executed and remained passing.

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Codex Task