#29792 fix(next): stop transcript activity jolts and remove running pill

Leo called out that the transcript “keeps jolting every time there’s a new subagent or command,” looks terrible and misaligned, and asked why the bottom pill exists. This PR keeps live activity arrivals anchored and removes the duplicate running-activity pill above the composer.

Activity and running-tray rows now use opacity-only entry motion, so their measured space is reserved without translating the visible transcript. The old AgentsPill mount and its dock anchor are gone; the tray remains the single running-work surface. Coverage checks the activity-row fade, the reader anchor after an appended activity row, and that a running chat has no second pill. The chat-transcript design spec records the settled rule.

Validation: git diff --check; fleet webview tests will run on the PR head.

Agent session: agent session
🤖 Generated with Claude Code

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Summary by cubic

Stops the transcript from jolting whenever a subagent or command arrives, and removes the duplicate running-activity pill above the composer.

• Activity rows and running-tray arrivals now use opacity-only entry motion, so the visible transcript stays put when new work appears.
• Removed the AgentsPill and its dock anchor; the running tray is the single running-work surface, including when the tray debug setting is off.
• Coverage checks the activity-row fade, that an appended activity row keeps the reader anchored, and that no second pill ever renders.