feat(coding-agent): add ctx.getSystemPrompt() to extension context

Adds a method to access the effective system prompt (after any per-turn
extension modifications) from the extension context.

Implementation:
- Add systemPrompt getter to AgentSession reading from agent.state.systemPrompt
- Wire getSystemPrompt through ExtensionContextActions to ExtensionRunner
- Add getSystemPrompt to interactive-mode's shortcut context
- Update docs with ctx.getSystemPrompt() section
- Add system-prompt-header.ts example
- Add example to docs reference table

Closes #1098

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added `ctx.getSystemPrompt()` to extension context for accessing the current effective system prompt ([#1098](https://github.com/badlogic/pi-mono/pull/1098) by [@kaofelix](https://github.com/kaofelix))
+
 ## [0.50.5] - 2026-01-30
 
 ## [0.50.4] - 2026-01-30