feat(experimentalist): migrate agent context to Ethos

[aleckhoury]

Summary

This change applies the direct Agent Spec-to-Ethos rename across Experimentalist inputs, profiles, runtime components, prompts, tests, and documentation.
It preserves Experimentalist behavior while materializing the renamed contract as ETHOS.md.
This child PR targets the authoritative rename PR and depends on the Analyst profile rename.

Changes

• Rename the Experimentalist CLI, profile, resolver, runtime, and backend inputs to Ethos terminology.
• Rename existing optimization context, analyzer, scorer, and prompt references without changing their behavior.
• Materialize the renamed contract as ETHOS.md in experiment workspaces.
• Rename three example contracts and update their existing profiles, benchmarks, documentation, and tests.
• Remove prior runtime names and compatibility aliases.

Type of Change

• Code change (feature, bug fix, or refactor)
• Code change with documentation updates
• Documentation only
• Contributor tooling or automation
• CI, build, or test infrastructure

Quality Gates

• Tests added or updated for changed behavior
• Existing tests cover changed behavior — justification: existing Experimentalist tests use the renamed fields, flags, paths, and runtime inputs.
• Tests not applicable — justification:
• Documentation updated for user-visible behavior
• Documentation not applicable — justification:

Verification

• Pull request title follows the repository's Conventional Commit format
• Every commit includes an appropriate Signed-off-by: trailer
• uv run pre-commit run -a passes, or any blocked checks are identified below
• Targeted tests pass, or tests are marked not applicable above
• No secrets, API keys, or credentials are included

Targeted validation:

• Experimentalist tests layered with the Analyst profile rename: 928 passed, 41 skipped.
• uv run --frozen ruff check plugins/nemo-experimentalist: passed.
• uv run --frozen ruff format --check plugins/nemo-experimentalist: 129 files already formatted.
• PR-scoped uv run pre-commit run --files: passed, including the repository type-check hook.
• git diff --check: passed.
• uv run pre-commit run -a: core checks passed. The Helm hook modified unrelated generated documentation, and this worktree lacks Studio's lint-staged binary.
• A direct source-tree type check reports two pre-existing StrategyContext diagnostics and the sibling resolve_ethos_path() dependency.