Make map-spawned knife entities respawn like other weapons (#163)

[darkshade9]

Summary

• Map-placed weapon_knife entities now respawn after pickup using weapon_respawn timing, matching the behaviour of every other weapon pickup in the game.
• Player-dropped knives are unchanged — they continue to follow the existing dropped-item lifecycle and do not become permanent map respawners.
• The DF_WEAPONS_STAY DM flag is respected: when set, the knife entity gets FL_RESPAWN instead of a timed SetRespawn, exactly as grenades handle it.

Files modified

• src/action/p_weapon.c — added 7 lines to the KNIFE_NUM pickup branch inside Pickup_Weapon

Verification

Step	Command	Result
Lint/build	ninja -C builddir gamearm64.dylib	✅ pass — 0 new warnings (19 pre-existing in botlib, unrelated)
Smoke test	tests/e2e/startup.spec.ts	⚠ skipped — spec not present in this repo
Tests	no test runner detected	⚠ No test infrastructure; manual verification scenarios outlined in plan

Manual verification scenarios (per plan §4):

1. Map-placed knife can be picked up; entity disappears immediately.
2. Knife entity reappears after weapon_respawn seconds.
3. Pickup fails cleanly when player is at knife_max; entity state unchanged.
4. Player-dropped knives still clean up via the existing drop lifecycle.
5. Grenade pickup behaviour unaffected.

Commit tier

Tier 2b (local unsigned commit + push) — repo does not enforce signed commits on aqtion.

Notes

• Branch was created from master by the conductor (local origin/HEAD → master) but aqtion is the GitHub default branch. A merge commit bringing in origin/aqtion precedes the feature commit; this PR is targeted at aqtion.
• g_items.c needed no changes — the SetRespawn / FL_RESPAWN machinery is already correct and shared.
• Workspace mode: per-execute worktree at /Users/dino/Documents/git/q2pro/.prismconductor/worktrees/q2pro-163

Closes #163