feat(model): per-session context window override via /model

[F0Rextasy]

What

I reviewed the full diff; context length was startup-only config, so /model now accepts --context-window to override it mid-session for the current session only.

Why

Fixes #12578

Testing

• bun test packages/coding-agent/test/slash-commands/session.test.ts (5 pass, unaffected surface)
• bun --cwd=packages/coding-agent run check:types (pass)
• oxlint + oxfmt on the changed file (pass)

---

• bun check passes
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

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[anthonyalayo]

Hey there, thanks for adding this support! I filed the associated issue here: #12578

Regarding the UX, is there a way to also change the context window within the TUI session?