Add Standalone Activities feature reference

[donald-pinckney]

Summary

• Adds skill coverage for Standalone Activities (Public Preview) — Temporal's job-queue primitive that runs a single Activity as a top-level execution without a Workflow.
• Six new reference files: a core concept page, a temporal activity CLI reference, and per-SDK quickstarts for Python, Go, Java, and .NET.
• SKILL.md gains a new "Standalone Activities" section pointing at the new files and noting that TypeScript SDK coverage is upstream-pending (no develop/typescript/activities/standalone-activities.mdx exists yet).

Requires Temporal CLI v1.7.0+ and Server v1.31.0+. Public Preview in Temporal Cloud.

What's in the box

• references/core/standalone-activities.md — concept, when-to-use vs. Workflow, dual-use, conflict/reuse policies, Public Preview limitations, observability, billing framing
• references/core/standalone-activities-cli.md — every temporal activity subcommand under its own heading; flags Public Preview gaps for pause/reset/update-options
• references/{python,go,java,dotnet}/standalone-activities.md — language-specific quickstarts following each upstream doc's section structure

Authored grounded against temporalio/documentation with inline citations on every non-trivial claim. Methodology and remaining VERIFY markers are captured in AUTHORING_LOG.md; the authoring plan is in STANDALONE_ACTIVITIES_AUTHORING_PLAN.md.

Known gaps (documented as VERIFY markers, not invented)

• The four SDK docs do not yet show conflict-policy / reuse-policy parameter wiring (Python kwargs, Go fields, Java builders, .NET options). The encyclopedia lists the policy values (USE_EXISTING, REJECT_DUPLICATES; TerminateExisting / TerminateIfRunning unsupported in Public Preview) but the per-language API names need to be confirmed against godoc/javadoc/etc. before snippets are added.
• TypeScript SDK has no upstream Standalone Activities doc; not authored here.

Test plan

• Spot-check 5 random citations resolve to the cited doc lines
• Confirm Public Preview limitations (no TerminateExisting / TerminateIfRunning, no pause/reset/update-options) appear in both core file and CLI file
• Confirm version requirements (CLI v1.7.0+, Server v1.31.0+) appear consistently
• Confirm no TypeScript SDK file was added and SKILL.md flags the gap
• Render SKILL.md locally and confirm the new section sits cleanly above "Task Queue Priority and Fairness"

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