chore(deps): replace dgageot/ultraviolet fork with upstream

Goal

Remove the replace github.com/charmbracelet/ultraviolet => github.com/dgageot/ultraviolet ... directive and use the official upstream revision already required by go.mod:
github.com/charmbracelet/ultraviolet v0.0.0-20260811164956-006e29f97886.

Why this is now safe

The current replacement is pinned to github.com/dgageot/ultraviolet v0.0.0-20260313154905-9451997d56b6, reintroduced for the macOS/iTerm2 emoji-rendering regression tracked by #2089 (after the earlier #1233 issue).

A source comparison of the fork commit 9451997d56b6 against the exact upstream target 006e29f97886 found that upstream now contains a functionally equivalent or improved solution for every fork change:

• correct handling of wide-cell/emoji placeholders, with upstream regression coverage;
• iTerm2/wcwidth cursor-drift handling, improved via wide-line reanchoring and grapheme-width negotiation;
• safe colored-blank erase handling through explicit pen updates;
• an additional upstream fix for Kitty extended-key release/repeat events that remains absent in the frozen fork.

Docker-agent direct usages (DEC 2031 color-scheme events, EventDecoder, and NewCancelReader) have no identified behavioral incompatibility. Both modules have compatible dependency sets, MIT licensing, and upstream's Go version requirement is compatible with docker-agent's go 1.26.5.

Sandboxes precedent

/Users/arnaud/Workspace/Dev/docker/sandboxes commit cec304d2f (Stop using the ultraviolet fork, 2026-06-25) removed its dgageot replacement without a later revert or reported Ultraviolet regression. This is supporting evidence only: sandboxes has no direct Ultraviolet imports and does not independently validate docker-agent's iTerm2 path.

Implementation plan

1. Add targeted regression coverage for wide emoji/background rendering, and establish a green baseline against the current fork.
2. Run a manual blocking check of the TUI on macOS + iTerm2 with real emoji content; the original regression only had manual visual confirmation.
3. Remove the replace directive, retain the existing official upstream requirement, run go mod tidy, and update go.sum.
4. Remove the obsolete ultraviolet entry from .golangci.yml's gomoddirectives.replace-allow-list.
5. Validate with task lint, task test, task build, native Windows CI, and the license check.

Rollback

If a gate fails, revert only the replacement-removal change: restore the replace directive, matching go.sum updates, and the linter allow-list. The regression coverage can remain.

Open decision

Assign a maintainer/tester to perform the required macOS+iTerm2 manual check before the replacement-removal PR merges.