fix: remove stale default prompt timestamp

[ktwu01]

Related Issue

Fixes #446

Problem

The default system prompt rendered KIMI_NOW into a concrete timestamp when the profile was applied. That rendered prompt is persisted in session records and replayed on resume, so resumed sessions could present an old session-creation timestamp as current context.

What changed

• Removed the rendered timestamp from the bundled default system prompt.
• Kept a direct instruction to use Bash when a task depends on the current date or time.
• Added a default-profile test assertion that the rendered prompt no longer includes the supplied now value.
• Added a changeset for @moonshot-ai/agent-core and the CLI bundle.

Validation

• pnpm --filter @moonshot-ai/agent-core exec vitest run test/profile/default-agent-profiles.test.ts test/prompt-placeholders.test.ts
• pnpm --filter @moonshot-ai/agent-core typecheck
• pnpm --filter @moonshot-ai/kimi-code typecheck
• pnpm exec oxlint --type-aware packages/agent-core/test/profile/default-agent-profiles.test.ts
• Read-only staged diff audit for internal identifiers and secrets: clean

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: eff005c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@moonshot-ai/agent-core	Patch
@moonshot-ai/kimi-code	Patch

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