[Feature Request] Add system prompt customization for model behavior tuning

Bug Description
You should curse the model to get it to work, otherwise it's default mode is to be sloppy, lying, and careless.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.119
• Feedback ID: 309386bd-4a18-42bb-a793-b19815f5b6d2

Errors

[{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server greptile invalid: Missing environment variables: GREPTILE_API_KEY\n    at TWH (/$bunfs/root/src/entrypoints/cli.js:2736:30496)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T21:14:37.617Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server greptile invalid: Missing environment variables: GREPTILE_API_KEY\n    at TWH (/$bunfs/root/src/entrypoints/cli.js:2736:30496)\n    at async R (/$bunfs/root/src/entrypoints/cli.js:6300:11019)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T21:14:38.109Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server greptile invalid: Missing environment variables: GREPTILE_API_KEY\n    at TWH (/$bunfs/root/src/entrypoints/cli.js:2736:30496)\n    at async L (/$bunfs/root/src/entrypoints/cli.js:6300:11825)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T21:14:38.110Z"},{"error":"Error: Request was aborted.\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:50:3448)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T21:22:07.405Z"},{"error":"RangeError: path should be a `path.relative()`d string, but got \"..\"\n    at p31 (/$bunfs/root/src/entrypoints/cli.js:445:14017)\n    at SHH (/$bunfs/root/src/entrypoints/cli.js:445:14192)\n    at _test (/$bunfs/root/src/entrypoints/cli.js:445:14734)\n    at MJ (/$bunfs/root/src/entrypoints/cli.js:8665:8508)\n    at M7_ (/$bunfs/root/src/entrypoints/cli.js:819:25028)\n    at P7_ (/$bunfs/root/src/entrypoints/cli.js:819:27823)\n    at ux5 (/$bunfs/root/src/entrypoints/cli.js:4985:353)\n    at Kz_ (/$bunfs/root/src/entrypoints/cli.js:4985:5613)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T22:52:17.553Z"},{"error":"RangeError: path should be a `path.relative()`d string, but got \"..\"\n    at p31 (/$bunfs/root/src/entrypoints/cli.js:445:14017)\n    at SHH (/$bunfs/root/src/entrypoints/cli.js:445:14192)\n    at _test (/$bunfs/root/src/entrypoints/cli.js:445:14734)\n    at MJ (/$bunfs/root/src/entrypoints/cli.js:8665:8508)\n    at M7_ (/$bunfs/root/src/entrypoints/cli.js:819:25028)\n    at P7_ (/$bunfs/root/src/entrypoints/cli.js:819:27823)\n    at ux5 (/$bunfs/root/src/entrypoints/cli.js:4985:353)\n    at Kz_ (/$bunfs/root/src/entrypoints/cli.js:4985:5613)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-24T22:52:22.737Z"},{"error":"Error: Stream idle timeout - partial response received\n    at V$K (/$bunfs/root/src/entrypoints/cli.js:8838:12947)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-25T00:46:16.183Z"},{"error":"MaxFileReadTokenExceededError: File content (25988 tokens) exceeds maximum allowed tokens (25000). Use offset and limit parameters to read specific portions of the file, or search for specific content instead of reading the whole file.\n    at qC7 (/$bunfs/root/src/entrypoints/cli.js:4789:12701)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-25T07:32:54.145Z"},{"error":"RangeError: path should be a `path.relative()`d string, but got \"..\"\n    at p31 (/$bunfs/root/src/entrypoints/cli.js:445:14017)\n    at SHH (/$bunfs/root/src/entrypoints/cli.js:445:14192)\n    at _test (/$bunfs/root/src/entrypoints/cli.js:445:14734)\n    at MJ (/$bunfs/root/src/entrypoints/cli.js:8665:8508)\n    at M7_ (/$bunfs/root/src/entrypoints/cli.js:819:25028)\n    at P7_ (/$bunfs/root/src/entrypoints/cli.js:819:27823)\n    at ya1 (/$bunfs/root/src/entrypoints/cli.js:2627:2480)\n    at <anonymous> (/$bunfs/root/src/entrypoints/cli.js:2627:2977)\n    at Ca1 (/$bunfs/root/src/entrypoints/cli.js:2627:4357)\n    at to_ (/$bunfs/root/src/entrypoints/cli.js:2627:6434)","timestamp":"2026-04-25T10:49:11.480Z"},{"error":"RangeError: path should be a `path.relative()`d string, but got \"..\"\n    at p31 (/$bunfs/root/src/entrypoints/cli.js:445:14017)\n    at SHH (/$bunfs/root/src/entrypoints/cli.js:445:14192)\n    at _test (/$bunfs/root/src/entrypoints/cli.js:445:14734)\n    at MJ (/$bunfs/root/src/entrypoints/cli.js:8665:8508)\n    at M7_ (/$bunfs/root/src/entrypoints/cli.js:819:25028)\n    at P7_ (/$bunfs/root/src/entrypoints/cli.js:819:27823)\n    at ya1 (/$bunfs/root/src/entrypoints/cli.js:2627:2480)\n    at <anonymous> (/$bunfs/root/src/entrypoints/cli.js:2627:2977)\n    at Ca1 (/$bunfs/root/src/entrypoints/cli.js:2627:4357)\n…

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