feat(context): compose context files instead of first-match-wins

[dlkakbs]

What does this PR do?

build_context_files_prompt() used an or-chain that silently dropped every context file after the first match. A project with both .hermes.md andAGENTS.md would only ever load .hermes.md -AGENTS.md was never seen by the agent.

This PR makes context files composable: all present primary sources are loaded and concatenated. Each file keeps its own independent 20k char cap rather than distributing a shared budget proportionally (#2846)This avoids penalizing large files when fewer sources are present - a project with only two context files still gets up to 20k each, not 10k.

.cursorrules remains a pure fallback and is only loaded when none of the primary sources are present.

Related Issue

Fixes #2835

Type of Change

• New feature (non-breaking change that adds functionality)

Changes Made

• agent/prompt_builder.py — replaced or-chain in build_context_files_prompt() with explicit composition; .cursorrules loaded only when no primary files are present
• tests/agent/test_prompt_builder.py — updated 3 tests that asserted first-match-wins behavior; added composition assertions

How to Test

1. Create a project with both .hermes.md and AGENTS.md
2. Run hermes in that directory
3. Verify both files appear in the system prompt context
4. Create a project with only .cursorrules (no .hermes.md, AGENTS.md, orCLAUDE.md)
5. Verify .cursorrules still loads as before

Checklist

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (feat(context):)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run pytest tests/ -q and all tests pass (95 passed, 1 pre-existing failure unrelated to this change)
• I've added tests for my changes
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant docstrings — updatedbuild_context_files_prompt() docstring to reflect new behavior
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact — N/A (pure Python path handling, no OS-specific code)
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

[teknium1]

Thanks for the contribution, @dlkakbs!

Closing this as a duplicate of #2846 (by @Mibayy), which targets the same fix/feature. We're consolidating on that PR for review.

If you want to help push it over the line, please jump in there — or if you think your approach is better for a specific reason that isn't covered in the other PR, let us know and we can reopen.