fix: Default frecency/history DB paths to ~/.pi/agent/fff/ (#474)

[gustav-fff]

Closes #474

Root cause

packages/pi-fff/src/index.ts:305-312 defaulted frecencyDbPath and historyDbPath to undefined. At the FFI boundary (packages/fff-node/src/finder.ts:112-113), undefined becomes empty string. In crates/fff-c/src/lib.rs:210-211, optional_cstr("") returns None. When None, frecency/query-tracker init skipped (lines 219-248), so both stay default uninitialized.

Fix

Default both to ~/.pi/agent/fff/frecency and ~/.pi/agent/fff/history, matching README claim (line 152). Flag/env overrides unchanged.

Steps to reproduce

Pre-fix (main):

# Install pi-fff without flags
pi install -l npm:@ff-labs/pi-fff

# Use fffind, select results
pi -c 'fffind main'

# Verify no LMDB created
ls ~/.pi/agent/fff/  # directory doesn't exist

# Check health
pi -c '/fff-health'  # shows "Frecency: disabled" and "Query tracker: disabled"

Expected: frecency/history active, LMDB files created under ~/.pi/agent/fff/.
Actual: both disabled, no files created.

Post-fix (this PR):

# Same steps
pi install -l npm:@ff-labs/pi-fff
pi -c 'fffind main'
ls ~/.pi/agent/fff/  # frecency/ and history/ exist

# Check health
pi -c '/fff-health'  # shows "Frecency: active" and "Query tracker: active"

How verified

• Reviewed diff: import statements added, default paths set before flag/env resolution.
• Checked make lint: no errors in pi-fff.
• Logic verified: path.join(os.homedir(), ".pi", "agent", "fff") constructs correct base, then frecency / history appended.
• Did not run live test (no pi install available in triage env), but logic straightforward: replaces undefined with concrete path.

Automated triage via gustav-fff bot.