fix(security): migrate MCP private-key handoff to keyfile pattern

[jewoos2921]

Summary

Previously, BUZZ_PRIVATE_KEY and NOSTR_PRIVATE_KEY were passed to MCP subprocesses via environment variables (PASSTHROUGH_ENV allowlist in buzz-agent). Any configured MCP server could read the user's identity key from its own environment — a known trust-boundary tradeoff documented in commit fbce606fe.

This PR migrates to a keyfile pattern so raw private keys never appear in subprocess environments.

Changes

buzz-dev-mcp/src/shim.rs — core fix

• Writes BUZZ_PRIVATE_KEY to a 0600 keyfile inside the shim's 0700 tempdir (same proven pattern already used for NOSTR_PRIVATE_KEY)
• Exposes the path via BUZZ_KEYFILE env var to shell children
• Removes BUZZ_PRIVATE_KEY from the process env immediately after writing the keyfile
• New Shim.buzz_env field carries BUZZ_KEYFILE + BUZZ_RELAY_URL (relay URL is non-secret)

buzz-cli/src/lib.rs — keyfile support

• New --keyfile flag / BUZZ_KEYFILE env var that reads the private key from a file
• Takes precedence over --private-key / BUZZ_PRIVATE_KEY
• Backward compatible — existing env-var usage still works for standalone CLI invocations

buzz-dev-mcp/src/view_image.rs — media auth

• Relay media Blossom t=get auth now reads from BUZZ_KEYFILE first, falling back to BUZZ_PRIVATE_KEY for backward compatibility

buzz-dev-mcp/src/shell.rs

• Passes buzz_env (BUZZ_KEYFILE, BUZZ_RELAY_URL) to shell children alongside git_env
• Bootstrap hint checks BUZZ_KEYFILE instead of BUZZ_PRIVATE_KEY

buzz-agent/src/mcp.rs — allowlist lockdown

• Removes BUZZ_PRIVATE_KEY and NOSTR_PRIVATE_KEY from PASSTHROUGH_ENV
• Adds BUZZ_KEYFILE (a filesystem path, not a secret)
• MCP subprocesses can no longer read identity keys from their environment

Security impact

Before	After
Any MCP server can read BUZZ_PRIVATE_KEY from its env	MCP servers only see BUZZ_KEYFILE (a path)
Key in env = readable via /proc/self/environ, env, etc.	Key in 0600 file inside 0700 tempdir, cleaned up on exit
NOSTR_PRIVATE_KEY already keyfile-isolated	BUZZ_PRIVATE_KEY now also keyfile-isolated

Verification

• cargo check — all 3 crates compile
• cargo clippy -- -D warnings — clean
• cargo fmt --check — clean
• cargo test -p buzz-cli -p buzz-dev-mcp -p buzz-agent --lib — 286 passed (2 pre-existing failures on main, unrelated to this change)