APP-97: Fix Truffile setup and inference guidance

[temp-deepshard]

Why

An agent following the docs literally hit avoidable failures: the uv path invoked a missing pip, the inference flag was stale, raw inference lacked host discovery and Python examples, and scripted deletion was unsafe.

Changes

• uses uv pip --python .venv/bin/python for uv environments
• uses the virtual environment's Python directly for the standard venv fallback
• verifies the installed CLI through .venv/bin/truffile
• documents device-host discovery and a minimal Python httpx inference client
• corrects --no-tools to --no-default-tools
• documents foreground, background, and hybrid app creation
• changes deletion to list, dry-run, human confirmation, and explicit --yes

Validation

• exact pushed commit fcdedfe832f8150aed4d48aebd80cb56d17b5473
• official Mint strict validation passed
• Python inference example parses
• command examples match the isolated Python 3.12 wheel test
• APP-96's real private-source wheel build and fresh-install verification passed in run 29476643574
• automated CI: none configured in this docs repository; validation was run manually

Depends on: deepshard/truffile#46 and deepshard/truffile#47

Followed by: #5

Linear: APP-97