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

[Mibayy]

Fixes #2835

Problem

build_context_files_prompt() used an or-chain that silently dropped every context file after the first match. A project with both .hermes.md and AGENTS.md would never see the second one.

Solution

Load all present primary sources and concatenate them. A shared 20,000-char budget is distributed proportionally when the combined content would exceed it.

New composition order:

Source	Role	Behavior
.hermes.md / HERMES.md	Per-project user instructions	Always loaded if present
AGENTS.md / agents.md	Recursive dev guide	Always loaded if present
CLAUDE.md / claude.md	Claude-specific instructions	Always loaded if present
.cursorrules / .cursor/rules/	Legacy fallback	Only loaded when none of the above are present

Backward compatibility

• Projects with a single context file type: identical behavior
• Cursor-only projects (.cursorrules, no AGENTS.md): identical behavior
• Only change: projects with multiple primary types now get all of them

[teknium1]

Thanks for the focused proposal. The reported behavior is still present on current main: build_context_files_prompt() uses the first-successful loader chain at agent/prompt_builder.py:1976-1982.

Problems

• This is a reversal of an intentional design, not merely an omitted composition step. Commit 2da79b13dfae6476c8a0c268b2ea9ac7cd91a665 explicitly replaced prior composition because it could bloat the system prompt with redundant or conflicting instructions. Maintainer agreement is needed on the new conflict-resolution contract.
• The proposed fixed 20,000-character aggregate cap does not preserve current main's cap contract. agent/prompt_builder.py:1201-1219 resolves an explicit configured cap or a model-window-derived dynamic cap, and agent/system_prompt.py:451-453 passes context_length into the builder.
• The PR changes documented behavior but omits updates to website/docs/user-guide/features/context-files.md:109, website/docs/user-guide/configuration.md:1973, and website/docs/developer-guide/prompt-assembly.md:188.

Suggested changes

• Preserve the current context_length flow and resolve one aggregate budget through _get_context_file_max_chars(context_length).
• Add aggregate-budget tests for explicit configuration and dynamic-cap cases, then update the affected documentation.

Automated hermes-sweeper review.

[teknium1]

Current main no longer has a fixed context-file cap: agent/prompt_builder.py:1201-1219 resolves context_file_max_chars or a model-window-derived budget, and the builder receives context_length. A salvage should preserve that flow and derive the aggregate budget through _get_context_file_max_chars(context_length) rather than hard-coding 20,000.