What does this PR do?

First pass draft implementation at loading of rules from subdirectories.

Closes #6316

Working

• Loading of subdirectory rules in response to tool calls that introduce files which should result in a rule load
• Path filtering via YAML frontmatter, meaning that rule application can be restricted further to only apply when files matching the globs are in context.
• Configuration under subdirectoryRules option
  • Disabled by default
  • Glob patterns can be set via patterns, default is **/AGENTS.md
  • Searches up by default, can be configured to just load rule files that are in the same directory as the loaded file by setting the exact option to true
• When tool calls that introduced rules into the session are pruned, the rules are also pruned. Tool calls that would match a rule are all tracked, so any live tool calls that reference a rule will keep the rule loaded into the session
• TUI shows rule loads in the message interface, all currently loaded rules are listed in the sidebar

Not working

• Web UI + Desktop UI (Rule loading triggers, just having trouble getting the UI aspect working)

How did you verify your code works?

• Running the TUI, adding additional AGENTS.md files, sending chat messages that should trigger loading of rules
• Simple unit tests