docs(har): document HAR truncation signal and default bump

[asensagent]

Summary

• Syncs HAR truncation envelope behavior (ACT-970, PR feat(har): bump DEFAULT_MAX_ENTRIES to 10000 + surface truncation signal in stop envelope (ACT-970) #583) across all four doc surfaces
• Documents --max-entries default bump from 2000 to 10000
• Documents truncation signal: meta.truncated, meta.warnings with HAR_TRUNCATED message, data.max_entries
• Documents clean stop behavior: meta.truncated = false, empty meta.warnings
• Documents har stop return shape: { path, count, dropped, max_entries }

Surfaces updated

• skills/actionbook/references/command-reference.md — --max-entries default, har stop return shape, truncation signal section
• docs/api-reference/cli.mdx — --max-entries default, Note block with truncation envelope details
• docs/guides/browser.mdx — --max-entries default, Tip on truncation detection
• skills/actionbook/SKILL.md — new "HAR Recording" section with truncation guidance

Source of truth

• packages/cli/src/browser/observation/network_har.rs — DEFAULT_MAX_ENTRIES = 10000, execute_stop emits __truncated + __warnings
• packages/cli/src/output.rs:31-48 — envelope bridge for meta.truncated / meta.warnings

Test plan

• Verify command-reference.md truncation signal section renders correctly
• Verify cli.mdx Note block renders in docs site
• Verify browser.mdx Tip renders in docs site
• Verify SKILL.md HAR Recording section parses correctly as skill context
• Cross-check default (10000), warning message format, and field names against source of truth