docs: add persistent memory guide

[hieptl]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes
Companion docs for OpenHands/software-agent-sdk#4178 (SDK issue #2037), which adds opt-in persistent memory to the SDK. Review on that PR asked for an example and docs: the example ships there (examples/01_standalone_sdk/55_persistent_memory.py); this PR adds the documentation.

• New guide: sdk/guides/persistent-memory.mdx — covers enabling the feature (AgentContext(load_memory=True), opt-in and off by default); the two memory tiers (user ~/.openhands/memory/, project <workspace>/.openhands/memory/); the file model (MEMORY.md curated index injected at session start vs. daily logs read on demand); what gets injected (the <MEMORY_CONTEXT> block, <UNTRUSTED_CONTENT> wrapping, ~6,000-character truncate-from-top budget, user tier before project tier); operational properties (re-resolved each session, excluded from base_state.json and API payloads, best-effort loading); and the maintenance habits the system prompt teaches the agent. Ends with the standard Ready-to-run Example section (<RunExampleCode> + GitHub link) embedding examples/01_standalone_sdk/55_persistent_memory.py.
• Navigation: registers sdk/guides/persistent-memory in docs.json under SDK → Guides, right after Context Condenser.

Verification

• The embedded example block is byte-in-sync with the SDK file per this repo's own sync logic (.github/scripts/sync_code_blocks.py extract_code_blocks + normalize_content comparison).
• docs.json re-parses as valid JSON after the nav edit.
• All internal links point at existing pages (/sdk/guides/skill, /sdk/guides/context-condenser, /sdk/guides/convo-persistence, shared snippet /sdk/shared-snippets/how-to-run-example.mdx), and the #ready-to-run-example anchor matches its heading.

Notes

• Merge after feat: add opt-in persistent memory across sessions software-agent-sdk#4178: the GitHub link targets the example on main, and the sync-docs-code-blocks workflow reads example content from the SDK repo's main. Until then the sync workflow logs a missing-source warning for this block and skips it (not a CI failure), and the PR-level check-broken-links job only checks internal links, so CI on this PR is unaffected.
• llms.txt/llms-full.txt are intentionally untouched: regenerating now bundles unrelated drift from previously merged pages (e.g. the Fork-a-Conversation guide), and the scheduled check-llms-files.yml workflow owns that sync.

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[neubig]

LGTM, thank you!