feat: avoid duplicate project-map hint injection by checking context

The before_agent_start handler now inspects the current session context
and skips injection if a pi-project-map-hint custom message is already
present in the active branch. This prevents duplicate hints on every
prompt while still re-injecting after compaction or tree navigation.

Also removes the unused hooks/on-prompt.ts prompt-text injector.

design-doc.md

@@ -291,8 +291,6 @@ pi-project-map/
 │   ├── merge.ts             # Merge AST + LLM outputs
 │   ├── format.ts            # Dense markdown formatter
 │   └── config.ts            # Skill configuration (thresholds, ignore patterns)
-├── hooks/
-│   └── on-prompt.ts         # Injects maintenance command into prompts
 └── README.md                # Setup and usage for humans
 ```
 
@@ -303,8 +301,9 @@ pi-project-map/
 - `project-map:reinit [path]` — Force re-initialization of entire project or subtree.
 
 ### Prompt Hook
-- On every prompt, the skill appends a lightweight instruction:
+- On each prompt, the skill injects a lightweight custom message **only if it is not already present in the current branch of context**:
   > "If you modify any source file, run `project-map:patch <path>` to update the analysis. If you suspect staleness, run `project-map:validate`."
+- The extension checks `ctx.sessionManager.buildSessionContext()` for an existing `pi-project-map-hint` custom message and skips injection when one is found. This prevents duplicate hints after steering, follow-up messages, or multi-turn conversations. The hint is automatically re-injected after compaction or `/tree` navigation removes it from the active path.
 
 ## 9. Risks and Tradeoffs