[issue-7014] Replace xterm fit addon with in-tree helper

[atomantic]

Summary

• Replace the micro-package @xterm/addon-fit with the in-tree fitTerminal helper.
• Preserve xterm's sizing behavior, including CSS padding, overview-ruler allowance, minimum geometry, and render invalidation.
• Keep initial-frame, window-resize, fullscreen, and container-observer refits wired through the shell session hook.
• Remove the dependency from the client manifests and record it as removed in docs/DEPS.md.

Test plan

• npm test -- src/lib/terminalFit.test.js src/lib/index.test.js src/hooks/useShellSession.test.jsx — 40 passed
• npm run lint — passed
• npm run build — passed
• Full client suite — 11,444 passed, 7 skipped; one unrelated existing VideoTimelineEditor loading-state test failed while its localhost:3000 service was unavailable

Closes #7014

[atomantic]

Required code review was not completed before publication. This PR is intentionally left open and will not be merged until the required review completes.

[atomantic]

Review-loop status: Antigravity (gemini-3.8-flash, optional, maximum one round) is inconclusive (no-verdict, zero review invocations). command -v agy confirmed the binary is installed; its installed --help provides neither an invocation-local isolated-settings selector nor an enforced empty tool set with MCP/hooks/plugins disabled. No PR content was supplied to an unrestricted reviewer, and no substitute review was performed. This follow-up explicitly permits an inconclusive optional review to proceed. No existing review findings or unresolved review threads were present. The original CI gates failed because selected test jobs were cancelled; the complete CI run has been restarted and must pass before merge.