docs: retire IntelligentContext from README and installation guide

[bhavyachopra99]

Description

Public README and installation guide still marketed IntelligentContext and score-based history dropping after PR-B1 retired those stages in favor of live-zone-only compression. This updates the two first-touch docs so new users see the current pipeline: compress fresh tool output and new turns only; frozen prefix preserved; history never dropped.

Closes #1444

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• README.md — replace IntelligentContext marketing bullet with live-zone compression (new bytes only; frozen prefix preserved; history never dropped)
• README.md — pipeline internals list current transforms and note IntelligentContext / RollingWindow retirement (PR-B1)
• docs/content/docs/installation.mdx — core package description matches live-zone ContentRouter
• docs/content/docs/installation.mdx — add PR-B1 retirement note for IntelligentContext / RollingWindow

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

$ rg -n 'IntelligentContext' README.md docs/content/docs/installation.mdx
README.md:289:- **Transforms** do the work: ... (live-zone only; IntelligentContext and RollingWindow were retired in PR-B1).
docs/content/docs/installation.mdx:31:> **Note:** IntelligentContext / RollingWindow ... were retired in PR-B1.

$ rg -n 'live-zone|Live-zone' README.md docs/content/docs/installation.mdx
README.md:274:- **Live-zone compression** — compresses only new bytes ...
README.md:289:- **Transforms** do the work: ... (live-zone only; ...)
docs/content/docs/installation.mdx:29:The core package includes ... live-zone ContentRouter compression.

Real Behavior Proof

• Environment: macOS (darwin 25.5.0), branch docs/retire-intelligentcontext-readme in /Users/bhavya/Desktop/Headroom-upstream
• Exact command / steps: rg -n 'IntelligentContext' README.md docs/content/docs/installation.mdx and rg -n 'live-zone|Live-zone' README.md docs/content/docs/installation.mdx; read updated README pipeline section and installation.mdx core-package blurb
• Observed result: IntelligentContext appears only in retirement notes (not as an active feature); live-zone compression is the primary marketed behavior in README and installation guide
• Not tested: Wiki pages (tracked as follow-up in Docs: README and public docs still market retired IntelligentContext / history dropping #1444); docs site build (npm run build in docs/)

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Additional Notes

Wiki still has extensive IntelligentContext docs — out of scope here; follow-up tracked in #1444. CHANGELOG N/A (docs-only, no release note required).

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.

[JerrettDavis]

Reviewed the README and installation guide updates. The active-feature language now matches the live-zone pipeline and keeps IntelligentContext/RollingWindow only as retirement context. Docs-only change with green governance checks; ready from my side.

[JerrettDavis]

Reviewed the post-merge docs diff. The README and installation guide still describe the current live-zone compression behavior accurately: fresh bytes/new turns are compressed, frozen prefixes stay cache-stable, and IntelligentContext/RollingWindow are only mentioned as retired history-dropping behavior. Docs-only change; no blocker.