refactor(vscode): remove dead code from Agent Manager

[marius-kilocode]

Static analysis (tsc unused-locals sweep + protocol message flow tracing) surfaced several Agent Manager code paths that are unreachable in production. This PR deletes them, net -316 lines.

Dead message flows (never travel the wire):

• agentManager.externalWorktrees: the extension posted it, but the webview has no handler. The webview also never sends requestExternalWorktrees, importExternalWorktree, or importAllExternalWorktrees, so the whole flow is dead: provider handlers, WorktreeImporter.external()/path()/all(), WorktreeManager.listExternalWorktrees(), ExternalWorktreeItem/ExternalWorktreeInfo, and the matching message type definitions on both sides of the protocol. Branch/PR import (importFromBranch/importFromPR) is untouched.
• agentManager.sessionMeta: posted after worktree creation but never handled in the webview. The worktreeSetup ready message already carries everything the webview consumes.

Dead interface surface:

• Host.serverPort(): no callers (the panel gets the port through buildWebviewHtml directly).
• SessionTerminalManager.hasTerminal()/hasActiveTerminal() and SetupScriptService.getScript(): no callers and no behavior tests.
• Unused imports (Disposable, ExtensionMessage, Popover) and an unused derivedFallbackBranches parameter.

Arch tests that asserted on the removed structure were updated to match.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Verified via grep that all removed symbols (ExternalWorktreeItem/ExternalWorktreeInfo, agentManager.externalWorktrees/sessionMeta/requestExternalWorktrees/importExternalWorktree/importAllExternalWorktrees, listExternalWorktrees, WorktreeImporter.external()/path()/all(), Host.serverPort(), SessionTerminalManager.hasTerminal()/hasActiveTerminal(), SetupScriptService.getScript()) have no remaining references anywhere in packages/kilo-vscode. Arch/unit tests were updated to match the removed code paths. No changeset needed since this is a pure internal dead-code removal with no user-facing behavior change.

Files Reviewed (15 files)

• packages/kilo-vscode/src/agent-manager/AgentManagerProvider.ts
• packages/kilo-vscode/src/agent-manager/SessionTerminalManager.ts
• packages/kilo-vscode/src/agent-manager/SetupScriptService.ts
• packages/kilo-vscode/src/agent-manager/WorktreeManager.ts
• packages/kilo-vscode/src/agent-manager/__tests__/AgentManagerProvider.spec.ts
• packages/kilo-vscode/src/agent-manager/host.ts
• packages/kilo-vscode/src/agent-manager/types.ts
• packages/kilo-vscode/src/agent-manager/vscode-host.ts
• packages/kilo-vscode/src/agent-manager/worktree-importer.ts
• packages/kilo-vscode/tests/unit/agent-manager-arch.test.ts
• packages/kilo-vscode/tests/unit/session-terminal-manager.test.ts
• packages/kilo-vscode/webview-ui/agent-manager/AgentManagerApp.tsx
• packages/kilo-vscode/webview-ui/src/types/messages/agent-manager.ts
• packages/kilo-vscode/webview-ui/src/types/messages/extension-messages.ts
• packages/kilo-vscode/webview-ui/src/types/messages/webview-messages.ts

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_{Reviewed by claude-sonnet-5 · Input: 34 · Output: 7.2K · Cached: 848.8K}

_{Review guidance: REVIEW.md from base branch main}