chore: snapshot working tree - pty_exited notifications + in-flight inference WIP

feat(booterm): structured pty_exited WS notifications. Plan-validated, impl-validated, code-reviewed green (contracts build clean, contracts test 29/29, booterm + web typecheck clean).

wip: in-progress inference/provider refactor (agents.ts, provider.ts, new llama-providers.ts, removed llama-args-validator), plus arena, dispatcher, compaction, schema changes.

openspec: pty-exit-notifications complete; x-agent-flags planned (not yet implemented).

openspec/changes/x-agent-flags/specs/agent-flags-header/spec.md

@@ -0,0 +1,46 @@
+## ADDED Requirements
+
+### Requirement: Agent llama_flags frontmatter field
+The system SHALL parse a `llama_flags` string field from agent AGENTS.md frontmatter.
+
+#### Scenario: Agent with llama_flags set
+- **GIVEN** an agent with `llama_flags: "--cache-type-k q8_0 -c 16384"`
+- **WHEN** the agent is parsed from AGENTS.md
+- **THEN** `agent.llama_flags` equals `"--cache-type-k q8_0 -c 16384"`
+
+#### Scenario: Agent without llama_flags
+- **GIVEN** an agent with no `llama_flags` field in frontmatter
+- **WHEN** the agent is parsed from AGENTS.md
+- **THEN** `agent.llama_flags` equals `null`
+
+### Requirement: X-Agent-Flags header emission
+The inference pipeline SHALL emit an `X-Agent-Flags` HTTP header when the agent has `llama_flags` set.
+
+#### Scenario: Header emitted for agent with flags
+- **GIVEN** an agent with `llama_flags: "--cache-type-k q8_0"`
+- **WHEN** `streamCompletion()` is called with that agent
+- **THEN** the `streamText()` call receives `headers: { 'X-Agent-Flags': '--cache-type-k q8_0' }`
+
+#### Scenario: No header when agent has no flags
+- **GIVEN** an agent with `llama_flags: null`
+- **WHEN** `streamCompletion()` is called with that agent
+- **THEN** no `X-Agent-Flags` header is included in the request
+
+#### Scenario: No header when agent is null
+- **GIVEN** no agent (raw chat session)
+- **WHEN** `streamCompletion()` is called
+- **THEN** no `X-Agent-Flags` header is included in the request
+
+#### Scenario: Whitespace-only flags produce no header
+- **GIVEN** an agent with `llama_flags: "   "`
+- **WHEN** `streamCompletion()` is called with that agent
+- **THEN** no `X-Agent-Flags` header is included in the request
+
+### Requirement: Existing sampler fields unchanged
+The existing sampler fields (top_k, min_p, etc.) SHALL continue to flow through `providerOptions.openaiCompatible` in the request body, independent of the `X-Agent-Flags` header channel.
+
+#### Scenario: Dual-channel sampling
+- **GIVEN** an agent with `top_k: 20` and `llama_flags: "--cache-type-k q8_0"`
+- **WHEN** an inference request is made
+- **THEN** the request body contains `top_k: 20` via providerOptions
+- **AND** the request header contains `X-Agent-Flags: --cache-type-k q8_0`