fix: agents update patches from full config so a rename can't wipe the pipeline

[rajarshidattapy]

What

Fixes #2

bolna agents update fetched the full agent (GetAgent) but then threw it away and PATCHed a single-key payload — e.g. {"agent_config":{"agent_name":"new"}} for a rename, or {"agent_prompts":{"task_1":{...}}} for --prompt. If the API replaces (or only shallow-merges the top-level agent_config/agent_prompts keys), that wipes everything else: tasks, voice, telephony, synthesizer, transcriber, LLM, and any sibling task_2 (summarization) prompt.

Fix

Extract a pure buildAgentUpdate(...) that starts from a copy of the agent's existing nested agent_config / agent_prompts and overlays only the changed fields, so the PATCH always carries the complete object — safe whether the API replaces or shallow-merges those keys.

Files:

• internal/api/agents.go — Agent.AgentConfig() / Agent.AgentPrompts() to read the current nested objects from a GET response.
• internal/cli/agents.go — buildAgentUpdate(...) replaces the inline partial-payload construction; shallow-copies so it never mutates the fetched agent.
• internal/cli/agents_test.go — pins the guarantee.

Tests

• Rename preserves tasks / llm / synthesizer / agent_welcome_message.
• --prompt preserves the task_2 prompt.
• Builder does not mutate the caller's fetched agent.
• No-op input yields an empty payload (never PATCH when nothing changed).

go build, go vet, go test ./... all pass.

Confidence / remaining blocker

• agent_prompts (task_2 preservation): confirmed solid — SystemPrompt() already reads current["agent_prompts"] and works, so the nested shape is known.
• agent_config: rests on the codebase's documented assumption (api/agents.go:34 — GET mirrors the {agent_config, agent_prompts} create shape). Made defensive: if GET omits nested agent_config, config-only edits degrade to the old partial (marked with a ponytail: comment).