Add runtime context placeholder support (app_context) across docs, fixtures, init, and tests

[PredictabilityAtScale]

Motivation

• Add explicit runtime context placeholder examples so shared system instructions and templates can consume render-time context values (e.g., {{ app_context }}).
• Ensure the CLI scaffolding, documentation, fixture prompts, and unit tests reflect and verify this usage pattern.

Description

• Updated README, docs (cli.md, composition.md, getting-started.md, prompt-format.md, providers.md) to show app_context as a runtime placeholder and explain mapping from --vars to {{ name }} keys.
• Updated example and fixture prompt files (fixtures/prompts/*) to declare app_context in context.inputs and to use {{ app_context }} in system and prompt template sections.
• Updated the init command (src/cli/commands/init.ts) scaffolding to include app_context in generated prompt, include file, and test sidecar examples.
• Extended interpolation tests (tests/interpolate.test.ts) and added new integration-style tests (tests/runtime-context-placeholders.test.ts) to verify {{ app_context }} interpolation across modes and environments (source-only and compiled-only).

Testing

• Ran the unit test suite with vitest, including tests/interpolate.test.ts and the new tests/runtime-context-placeholders.test.ts, and all tests passed.
• Verified that rendering in both source-only and compiled-only modes substitutes {{ app_context }} into system and template sections and selects environment overrides as expected.

---

Codex Task