feat: add unified agent conversations

[marcelormendes]

Summary

• add a provider-neutral conversation API with stable turn, message, plan, tool, file-change, approval, and attachment models
• add Codex, Claude, Pi, and Oh My Pi transcript adapters plus bounded live overlays
• add authenticated attachment staging, prompt submission, approval responses, subscriptions, and restored-pane identity continuity
• preserve compatibility for clients and panes that do not advertise conversation capability

Validation

• just check with pinned Rust 1.96.1
• 3,496 Rust tests passed, 1 skipped
• strict Unix and Windows-target Clippy passed
• UI hot-path, integration-asset, marketplace, and 98 maintenance tests passed
• live-tested through Herdr Desktop against a Pi conversation, including streaming tools, elapsed working state, image paste, approvals, scrolling, and degraded-mode behavior

Context

This is the engine half of the unified agent chat experience implemented and dogfooded in Herdr Desktop.

[kangal-bot]

Hi @marcelormendes, thanks for your interest in contributing.

Herdr does not accept unsolicited implementation pull requests from contributors who are not listed in .github/APPROVED_CONTRIBUTORS.

The pull request author is not an approved contributor.

If you encountered a reproducible bug, report the observed behavior through the bug issue template. A report does not reserve the work or authorize a pull request; accepted fixes are normally implemented by Herdr’s maintainer-controlled agents.

Feature requests, behavior changes, and other proposals belong in GitHub Discussions. Do not open an issue merely to justify an implementation that was already written.

If a maintainer explicitly wants this implementation, they can reopen the pull request. Reopening by anyone else will be closed again automatically.

See https://github.com/herdrdev/herdr/blob/master/CONTRIBUTING.md for the contribution policy.

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