Add extensible schedule action API

[chaptersix]

This experimental API layer lets a Schedule start either a workflow or a standalone activity and select an implementation-registered overlap policy. It adds the schedule-specific standalone activity template, CustomOverlapPolicy, generic execution identity and typed workflow/activity status, running-execution and action-kind projections, and terminal close time while preserving every legacy workflow field.

This is an experimental, unmerged stack. The server rejects conflicting, unknown, and action-unsupported selectors; the API accepts only a registered policy name and exposes no executable policy code or parameters.

Related drafts: generated Go API, Go SDK, and server layers for contracts, standalone activity execution, visibility, and integration coverage.

Dependencies and reproducible pins:

• Original proto contract commit: 181d052558a750032a1b95de96961ba9987e00fe
• This PR head, including regenerated HTTP schemas and conventional timeout lint annotations: 40766d34ea6c0d2e59c100c1cf20cdf4fb286fe4
• Generated Go API: 5fd10bc0a6cd7694373353cf6e00a0886a0b8167
• Go SDK: a6bed7c8ed3c4b90c467f003571c98b44ea8785b
• Validated server code snapshot: f7f30fc4be01f16ac838f6106f47484643eb4a40
• Final server integration head: d80de5800cde0845596277a1af957c731eb1c0a2

Validation:

• BUF_CACHE_DIR=/tmp/temporal-saa-buf-cache buf lint
• make api-linter
• make http-api-docs
• git diff --check