feat: add actionable config hints to all limit error messages

[simongdavies]

Every error message that fires when a configurable limit is breached now tells the LLM which config field to increase via manage_plugin.

fs-read (4 messages):

• maxFileSizeKb hint on all 'File too large' errors
• maxReadChunkKb hint on per-call chunk errors

fs-write (8 messages):

• maxWriteChunkKb hint on per-call chunk errors (text + binary)
• maxWriteSizeKb hint on cumulative write limit errors
• maxEntries hint on entry creation limit errors

fetch (9 messages):

• maxRequestsPerMinute/maxRequestsPerHour on rate limit errors
• maxDomainsPerSession on domain count errors
• maxDataReceivedKb on data budget errors
• maxResponseSizeKb on response too large errors
• maxRequestBodySizeKb on request body errors
• maxRedirects on redirect limit errors
• maxJsonResponseBytes/maxTextResponseBytes on convenience method errors

[Copilot]

Pull request overview

This PR updates limit-breach error messages across the built-in sandbox plugins to include actionable “which config key to increase” hints, intended to help an LLM resolve failures via manage_plugin.

Changes:

• Added config-field increase hints to fs-read limit errors (file size and per-call chunk size).
• Added config-field increase hints to fs-write limit errors (per-call chunk size, cumulative file size, and entry cap).
• Added config-field increase hints to fetch limit errors (rate limits, domain cap, data budget, response/body size caps, redirect budget, and convenience-method response caps).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
plugins/fs-write/index.ts	Adds config-field hints to fs-write limit errors for write/append and binary variants.
plugins/fs-read/index.ts	Adds config-field hints to fs-read limit errors for file size and per-call chunk size.
plugins/fetch/index.ts	Adds config-field hints to fetch rate-limit and size/budget limit errors (including convenience methods).