fix(agy-acp): Windows path resolution and sender_context noise stripping

[SunnyYYLin]

Closes

Closes #1329

At a Glance

agy-acp Adapter
│
├── Adapter::new()
│   └── HOME/USERPROFILE resolution
│       Windows: USERPROFILE first (openab sets HOME to workspace)
│       Unix:    HOME first, USERPROFILE fallback
│
├── agy_bin()
│   └── Windows: "agy" (PATH lookup)
│       Unix:    "/usr/local/bin/agy"
│
├── augmented_path()
│   └── Windows: ; separator, %USERPROFILE%\bin
│       Unix:    : separator, $HOME/bin
│
├── prepare_prompt_state()
│   └── Strip <sender_context>...</sender_context> before forwarding to agy
│
└── protobuf extraction
    └── Handle nested field(1) → field(20) → field(1) payload structure

Prior Art & Industry Research

Not applicable — this is a straightforward cross-platform compatibility fix for the Windows host environment.

Proposed Solution

1. HOME/USERPROFILE: On Windows, check USERPROFILE before HOME because the parent openab process sets HOME to the workspace directory, overriding the user's actual home. Applied in both Adapter::new() (conversation DB location) and augmented_path() (bin dir resolution).

2. agy_bin(): On Windows, use "agy" (system PATH lookup) instead of hardcoded /usr/local/bin/agy.

3. augmented_path(): Use ; separator on Windows, : on Unix. Query USERPROFILE for home-relative bin dirs on Windows.

4. sender_context stripping: Strip <sender_context>...</sender_context> XML blocks from prompts before forwarding to agy. CLI agents don't understand this metadata injected by the gateway (Issue tracking: Sender Identity Injection System for Agent Broker #61).

5. Nested protobuf extraction: Handle an additional nesting level in Gemini protobuf payloads where text is under field(20)→field(1)→field(1) instead of just field(20)→field(1).

Why This Approach

• Minimal cfg gates: Platform-specific logic is isolated behind cfg!(target_os = "windows") with clear fallbacks
• No breaking changes: Unix behavior is unchanged
• Correct priority on Windows: USERPROFILE first because HOME is polluted by the gateway
• Protobuf nesting: Non-destructive — tries the nested path first, falls through to the flat path

Alternatives Considered

1. Don't set HOME in openab parent: Would break child processes that expect HOME. Rejected — cleaner to fix the consumer.
2. Use a config field for home dir: Over-engineering. USERPROFILE is the standard Windows convention.

Validation

cargo check -p agy-acp --target x86_64-pc-windows-gnu  # clean
cargo check -p agy-acp                                  # clean
cargo clippy -p agy-acp -- -D warnings                  # no issues

[openab-app]

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[github-actions]

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