docs: add context management page for kagent 1.x (#503)

[boxcee-interview]

Closes #503

What was wrong

The 1.x doc set has no page about context management. kagent#2790 landed the CRD field and the translator path and shipped in v1.0.0-alpha2, so a reader can configure Harness.spec.kagent.compaction today with nothing in the docs to tell them how.

What the fix does

Adds docs-site/content/kagent/1.x/agents/context-management.md, the page the issue asks for:

• Opens by saying what compaction is and that it is Harness-level runtime policy, not an AgentTemplate setting.
• Carries a complete annotated manifest showing both strategies (sliding window and tail retention), plus a required/optional field table.
• States all four admission rules the API server enforces.
• Documents the summarizer-model revision behavior: a summarizer model other than the agent's own compiles a new revision, while naming the agent's own model is a deliberate no-op.
• Notes that compaction applies to the kagent runtime only, and links to the generated API reference for the field-by-field schema and to Agent harness, Agent memory, and Core concepts.

It also adds one cross-link from Agent harness's runtime-specific-settings section, so the two spec.kagent settings point at each other.

Interaction models (sync, async, events) are out of scope, per the issue.

How to Test

cd docs-site
hugo mod get ./...
hugo --config hugo.yaml -D --gc

Builds clean: 570 pages, no errors, no warnings. The new page assembles at /docs/kagent/1.x/agents/context-management/ and the Agent harness link resolves.