docs: add 10-minute quickstart guide with 3 runnable examples

[dgenio]

Summary

Create a dedicated 10-minute quickstart guide so new users can install contextweaver, run three working examples, and see tangible output with copy-paste commands.

Fixes #76

Changes

• docs/quickstart.md (new) — six-section quickstart guide:
  1. Prerequisites (Python version check, venv setup for Linux/macOS/Windows)
  2. Install (PyPI and editable-mode paths)
  3. First context build (ingest events → build_sync() → prompt + stats)
  4. Context firewall demo (large tool result → summary + artifact store)
  5. Tool routing demo (catalog → TreeBuilder → Router → focused shortlist)
  6. What to try next (links to existing docs + note about planned integration guides)
• README.md — added a prominent "10-Minute Quickstart" section linking to the new guide; fixed the context pipeline feature description from "seven-stage" to "eight-stage" (matching AGENTS.md and the actual pipeline)
• CHANGELOG.md — added entries under ## [Unreleased]

Checklist

• Tests added or updated for every new/changed public function
  • N/A — docs-only change, no public API modified
• make ci passes locally (fmt + lint + type + test + example + demo)
  • make unavailable on Windows; ran CI-equivalent commands individually:
    • ruff format --check src/ tests/ examples/ — 80 files already formatted
    • ruff check src/ tests/ examples/ — all checks passed
    • mypy src/ — no issues in 41 source files
    • pytest -q — 536 passed
    • All 7 example scripts — passed
    • python -m contextweaver demo — passed
• CHANGELOG.md updated under ## [Unreleased]
• Docstrings added for all new public APIs (Google-style)
  • N/A — no new public APIs
• Every modified module stays ≤ 300 lines (or a decomposition issue is linked above)
  • N/A — no source modules modified
• Related issue linked in the summary above
• Agent-facing docs updated if pipeline, API, or conventions changed
  • Reviewed AGENTS.md, .github/copilot-instructions.md, .claude/CLAUDE.md — no updates needed. The quickstart is user-facing, and the "seven→eight" fix in README aligns it with what AGENTS.md already documents.

Notes for reviewers

• Spec delta from issue docs: create 10-minute quickstart guide with 3 runnable examples #76: the issue's sample code used pack.token_count and pack.budget, which do not exist on ContextPack. The quickstart uses pack.stats.* fields that actually exist. The issue also linked to future framework integration guides not yet authored — the quickstart links to existing docs instead and notes integrations as planned separately.
• Expected output is shown as excerpts (not exact full dumps) so the guide stays accurate across minor formatting changes.
• Cross-platform: prerequisites include both source .venv/bin/activate (Linux/macOS) and .venv\Scripts\Activate.ps1 (Windows PowerShell).
• CI coverage: local validation ran on Windows with Python 3.14. CI will cover Ubuntu on Python 3.10/3.11/3.12.

[Copilot]

Pull request overview

Adds a dedicated quickstart entry point to help new users install contextweaver and run three end-to-end examples, and updates top-level docs to point to it and keep pipeline wording consistent.

Changes:

• Added docs/quickstart.md with a 10-minute guide and 3 runnable examples (context build, firewall, routing).
• Updated README.md to link prominently to the quickstart and corrected the pipeline stage count to eight.
• Updated CHANGELOG.md under [Unreleased] to note the new guide and README change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
docs/quickstart.md	New quickstart guide with install steps + runnable examples + next-step links.
README.md	Adds “10-Minute Quickstart” section and corrects pipeline stage count wording.
CHANGELOG.md	Documents the quickstart addition and README change under Unreleased.