feat(acp): --no-memory flag to disable NIP-AE core injection

[tlongwell-block]

Adds an operator opt-out for the ACP harness's NIP-AE agent core memory injection path, mirroring the existing --no-presence / --no-typing conventions.

Flag

• --no-memory / SPROUT_ACP_NO_MEMORY
• Default: off (memory injection enabled — current behavior is unchanged when the flag is not set)

Effect when set

• pool.rs: the per-session core engram fetch + decrypt + render is skipped entirely. No relay request, no timeout window.
• state.core_sections stays empty for every channel.
• format_prompt renders no [Agent Core Memory] section.
• Startup logs an info line at target engram::core so operators see the toggle is engaged.

What this flag does not touch

• The sprout mem CLI — unaffected, still works.
• The relay's acceptance of kind:30174 engrams — unaffected.
• The on-relay engram itself — never read, never overwritten.

This is purely a prompt-time injection toggle inside the ACP harness.

Why this shape

Tyler asked for one flag, --no-memory. The lookup itself already fails gracefully (transport / decrypt / parse / timeout → None, no section); this flag sits on top of that fallback so an operator can fully turn off the path even when it would otherwise succeed. Mirrors how --no-presence / --no-typing are wired (clap arg → *_enabled: bool on Config → field on PromptContext → gate at the call site).

Diff

file	change
crates/sprout-acp/src/config.rs	--no-memory clap arg + memory_enabled: bool field on Config (default true); included in summary(); updated test-helper default; 3 new unit tests
crates/sprout-acp/src/pool.rs	memory_enabled: bool on PromptContext; gate the is_new_session fetch block on ctx.memory_enabled
crates/sprout-acp/src/lib.rs	wire config.memory_enabled into the PromptContext builder; emit tracing::info! at startup when disabled

Total: 3 files, +74 / −2.

Verification

cargo fmt -p sprout-acp -- --check          # clean
cargo clippy -p sprout-acp --all-targets -- -D warnings   # clean
cargo test -p sprout-acp --lib              # 272 passed (269 existing + 3 new)
cargo run -p sprout-acp -- --help | rg no-memory          # flag visible

Behavior matrix

memory_enabled	result
true (default)	unchanged from PR #593 base: fetch with 3s bound, fail-open, optional onboarding nudge
false (--no-memory set)	no fetch, no section, no nudge — agent never sees [Agent Core Memory]

Targeting

Based on PR #593 (sami/nip-ae-minimal), since that PR has not yet merged to main. Should be merged into #593's branch before #593 lands, so the flag ships in the same release as the feature it gates.