fix(pi-fff): fall back to transient finder when LMDB env already open (#760)

[gustav-fff]

Closes #760

Root cause

heed 0.22 keeps a process-global OPENED_ENV registry (env_open_options.rs:405-422); opening the same canonical LMDB path twice in one process returns Error::EnvAlreadyOpened. pi-fff's main finder always opens the fixed ~/.pi/agent/fff/{frecency,history} (packages/pi-fff/src/index.ts:313-321), so an in-process subagent (createAgentSession) that loads the extension a second time hits this on FileFinder.create(). #700 fixed only aux finders, not the main finder.

Fix

When ensureFinder gets EnvAlreadyOpened from FileFinder.create(), retry once without frecencyDbPath/historyDbPath — a transient finder without persistent scoring, exactly like aux finders (#700). packages/pi-fff/src/index.ts.

Steps to reproduce

Rust layer (proves the engine-level cause), add to crates/fff-core/src/dbs/frecency.rs tests and run cargo test -p fff-search:

#[test]
fn repro_760() {
    let dir = std::env::temp_dir().join("fff_repro_760");
    let _ = std::fs::remove_dir_all(&dir);
    assert!(FrecencyTracker::open(dir.to_str().unwrap()).is_ok());   // first open
    let b = FrecencyTracker::open(dir.to_str().unwrap());            // second open, same process
    if let Err(e) = &b { eprintln!("{e}"); }
    assert!(b.is_ok());                                             // FAILS on pre-fix main
}

Pre-fix actual output:

SECOND OPEN ERROR: Failed to open frecency database env: environment already open in this program; close it to be able to open it again with different options

Expected: second open succeeds (or the caller degrades gracefully). In pi-fff this surfaces as FFF init failed: ...environment already open... on subagent start.

How verified

bun test test/     # packages/pi-fff -> 56 pass, 0 fail (adds a fallback test)

The new test falls back to a transient finder when the LMDB env is already open forces create() to return EnvAlreadyOpened once and asserts the retry drops the DB paths and session_start does not error.

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