docs: changelog for tool consolidation and enum renames (03-12-26) (#2941)

## Summary

- Adds changelog entry (`03-12-26-tool-consolidation-enum-renames.mdx`)
in the correct docs repo location (`docs/content/changelog/`)
- Covers two major Mercury PRs:
- [#17949](ComposioHQ/mercury#17949) — 1,545
enum slug renames for agent-friendly naming
- [#17466](ComposioHQ/mercury#17466) — 400+
overlapping tools consolidated, 52 high-usage tools deprecated with
pointers

Supersedes PR #2889 which was incorrectly opened in the fern repo.

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Author: Sushmithamallesh