agent/slack polish: announcement ordering for cancel-with-resource, UTF-16-safe label cap, stale spend list in base prompt

Low-severity residuals from the #3696 review, batched:

- `announcementFor` (slack-app/listeners/actions/proposal-button.js) checks the resource URL before `kind`, so a future cancel-kind op that also returns a resource would announce "Approved — follow it here" instead of "Cancelled". Latent today (cancel has no resource builder).
- `proposal-builder.js` caps the server-sent button label with `.slice(0, 75)` on UTF-16 units; an emoji at the boundary yields an unpaired surrogate that can make Slack reject the whole message. The file's own `capChars` is code-point-safe.
- The frozen base prompt's spend enumeration ("running a suite or a case, generating cases, cancelling a run") predates the two new gated ops. Fixing it is a deliberate cache-invalidating edit to the snapshot-pinned literal.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Read slack-app/listeners/actions/proposal-button.js and proposal-builder.js first, then locate the frozen base-prompt snapshot containing the spend enumeration. Verify the announcement decision is kind-aware, label truncation remains UTF-16-safe, and the snapshot lists the two newer gated operations without breaking its pinned wording.

Written by the indexing model from the issue text.