Follow-up polish: parseCatalogDocument side effects, StudioAiToolContext builder, read-only tools SPI

## Follow-up polish items (low priority)

These items were identified during the design review of #8 and remain as non-blocking polish after the refactor in #9.

### 1. `parseCatalogDocument()` side effects
Ensure the catalog parse method follows a pure parse→return pattern with no mutation of shared/static state during parsing. Any initialization of shared state should be deferred to an explicit `initialize()` or `load()` step.

### 2. `StudioAiToolContext` multi-argument constructor
Replace or supplement the raw multi-arg constructor with a proper Builder to eliminate parameter-order risks at call sites and improve readability.

### 3. Read-only tools allowlist drift
Derive read-only / pipeline-stage classification from the tool SPI itself (e.g., an `isReadOnly()` flag or `pipelineStage` field on each tool class) so that the manual allowlist in `AuthoringIntentRecipeEngine` cannot drift from the actual tool set.

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Raised during review of #9 by @coderabbitai.
Requested by @russdanner.