test(workspace-helpers): add unit tests + fix off-by-one in extractCustomAgentName

[rajbos]

Summary

Adds 36 unit tests for workspaceHelpers.ts, covering the pure string-transformation functions that determine session attribution (editor, repository, mode, MCP tools). Also fixes a pre-existing off-by-one bug discovered by the tests.

Bug fixed

extractCustomAgentName was calling slice(0, -10) to strip the .agent.md suffix, but that suffix is 9 characters, not 10. This silently dropped the last character of every custom agent name (e.g. "my-agent" became "my-agen"). Changed to slice(0, -9).

What's tested

• getModeType: null/undefined defaults, ask/edit/agent/plan/customAgent routing, plan-agent URI detection, custom .agent.md URI detection
• getRepoDisplayName: empty/unknown, HTTPS URLs, SSH URLs, git+https, GitHub Enterprise
• parseGitRemoteUrl: empty input, HTTPS remote, SSH remote, stops at next section boundary, returns undefined when no origin
• isMcpTool: mcp. and mcp_ prefixes, regular tool names
• normalizeMcpToolName: mcp_github_github_ -> mcp_io_github_git_, mcp.github.github. -> mcp.io.github.git., passthrough
• extractMcpServerName: toolNameMap lookup, known prefix fallbacks, generic fallback
• extractCustomAgentName: non-agent paths, file:/// URI, plain path
• getEditorNameFromRoot: empty, .copilot, Code Insiders, Cursor, .continue, opencode

Approach

All functions under test are pure string transformations. Tests use inline fixture strings -- no mocks, no file I/O.