feat: chunked exec streaming (exec_chunk) — remove 10MB output ceiling (#116)

[blaspat]

What

Adds chunked exec streaming (exec_chunk) — replaces the single 10MB-capped exec_result with incremental base64-encoded chunks sent as stdout becomes available. Removes the 10MB output ceiling entirely.

How

Capability negotiation: node advertises exec_chunk in hello.capabilities; server echoes back in hello_ack.capabilities. Only active when both sides support it. Falls back to current exec_result + 10MB cap otherwise.

Protocol (§3.7a):

• exec_chunk messages carry seq (monotonic), data (base64), more (true/false)
• Final chunk (more: false) carries exit_code, status, duration_ms, truncated
• Chunk size configurable via chunk_size in config.toml (default 1 MiB, range 4 KiB–10 MiB)

Go changes:

• messages.go — TypeExecChunk, ExecChunkPayload, NewExecChunkEnvelope
• HelloAckPayload — new Capabilities field
• client.go — store server caps, expose ServerCapabilities()
• config.go — ChunkSize field with clamped accessor
• shell.go — ChunkWriter callback in demuxer goroutine; flushes at chunk boundary
• handler_exec.go — dual-path: chunk streaming via WriteEnvelope or legacy exec_result; NoResponse field on Envelope signals dispatcher to skip write
• dispatch.go — NoResponse check
• main.go — wire WriteFunc, ChunkSize, SetServerCapabilities
• PROTOCOL.md — §3.7a spec + updated examples

Tests: 7/7 packages pass with -race.