docs(api): document Watch Management endpoints from #2110

[r266-tech]

Description

Documents the Watch Management surface introduced in #2110 (REST /api/v1/watches/*, the ov task watch CLI subcommand group, and the MCP list_watches / cancel_watch tools). All three control planes are net-new and currently undocumented in docs/{en,zh}/api/02-resources.md, where users land for everything watch_interval-related.

The new section follows the API documentation writing guide: Implementation Overview / Code Entry Points / Interface and Parameter Description / Usage Examples (HTTP + CLI + MCP). EN and ZH are mirrored.

Related Issue

Documents the changes from #2110 (RFC #2104 — Watch Management API).

Type of Change

• Documentation update

Changes Made

• docs/en/api/02-resources.md: insert a ### Watch Management section between ### add_resource and ### add_skill (~93 lines added)
• docs/zh/api/02-resources.md: mirrored Chinese section (~93 lines added)

Specifically captured:

• 5 REST routes (list / show / update / delete / trigger), each with both {task_id} and ?to_uri= variants
• PATCH /watches body schema with field semantics (notably watch_interval must be > 0; is_active orthogonal to the cadence for pause/resume; extra="forbid" → 422)
• ov task watch subcommands (ls / show / rm / pause / resume / update / trigger) with the <key> auto-classification (viking:// URI vs task_id)
• MCP minimum closure (list_watches, cancel_watch) and an explicit note that pause / resume / trigger / update are intentionally CLI/REST-only
• Cross-reference back to add_resource as the watch-creation entry point

Testing

• No code changes; docs-only PR
• Verified endpoint paths, verbs, body fields, validator messages, CLI subcommand names, and CLI flag names against openviking/server/routers/watches.py and crates/ov_cli/src/commands/watch.rs at HEAD
• EN/ZH parity check: same operations, same field semantics, same examples

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• My changes generate no new warnings

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