fix(task-board): cap a column automation's prompt length

[pedrofrxncx]

TASK_BOARD_AUTOMATION_UPSERT (landed same-day in #6690, apps/api/src/tools/task-board/automations.ts) took prompt as an unbounded z.string().nullable().optional(), writable directly by any org member's tool call into an unbounded text column — the same gap this codebase has repeatedly capped on sibling fields (task description/title, comment body, repo string), per the resource-bound hardening lane in the project history.

Why it matters: an org member (or a buggy caller) can write an arbitrarily large prompt into task_board_column_automations, which then gets fed into the Super Agent's instruction on every card landing in that column — unbounded row growth and an unbounded string re-sent on every automated run.

Fix: added MAX_AUTOMATION_PROMPT_LENGTH = 50_000 next to the existing MAX_TASK_DESCRIPTION_LENGTH/MAX_TASK_TITLE_LENGTH/MAX_TASK_REPO_LENGTH caps in schema.ts (same 50k bound as description/comment body), and applied it to the prompt field in TASK_BOARD_AUTOMATION_UPSERT's input schema.

Regression test: apps/api/src/tools/task-board/automations.test.ts, mirroring the existing description-length.test.ts pattern — asserts the schema rejects a prompt one character over the cap, accepts one at the cap, and still accepts null (the default-instruction sentinel).

To verify: bun test apps/api/src/tools/task-board/automations.test.ts.

Checked locally: bun run fmt, cd apps/api && bunx tsc --noEmit, the targeted test above, and bunx oxlint on the three touched files — all clean. Full CI validates the rest.

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Summary by cubic

Caps the prompt field in TASK_BOARD_AUTOMATION_UPSERT at 50,000 characters, matching the existing cap on task descriptions, so an org member's tool call can no longer store an arbitrarily large instruction that gets re-sent on every card landing.

• Adds a regression test covering the cap and the null default-instruction sentinel.
• No migration needed; existing prompts are unaffected.

^{Written for commit 080132c. Summary will update on new commits.}