feat: progressive subdirectory hint discovery (#5291)

As the agent navigates into subdirectories via tool calls (read_file,
terminal, search_files, etc.), automatically discover and load project
context files (AGENTS.md, CLAUDE.md, .cursorrules) from those directories.

Previously, context files were only loaded from the CWD at session start.
If the agent moved into backend/, frontend/, or any subdirectory with its
own AGENTS.md, those instructions were never seen.

Now, SubdirectoryHintTracker watches tool call arguments for file paths
and shell commands, resolves directories, and loads hint files on first
access. Discovered hints are appended to the tool result so the model
gets relevant context at the moment it starts working in a new area —
without modifying the system prompt (preserving prompt caching).

Features:
- Extracts paths from tool args (path, workdir) and shell commands
- Loads AGENTS.md, CLAUDE.md, .cursorrules (first match per directory)
- Deduplicates — each directory loaded at most once per session
- Ignores paths outside the working directory
- Truncates large hint files at 8K chars
- Works on both sequential and concurrent tool execution paths

Inspired by Block/goose SubdirectoryHintTracker.

website/docs/developer-guide/prompt-assembly.md

@@ -218,7 +218,7 @@ Local memory and user profile data are injected as frozen snapshots at session s
 `agent/prompt_builder.py` scans and sanitizes project context files using a **priority system** — only one type is loaded (first match wins):
 
 1. `.hermes.md` / `HERMES.md` (walks to git root)
-2. `AGENTS.md` (recursive directory walk)
+2. `AGENTS.md` (CWD at startup; subdirectories discovered progressively during the session via `agent/subdirectory_hints.py`)
 3. `CLAUDE.md` (CWD only)
 4. `.cursorrules` / `.cursor/rules/*.mdc` (CWD only)