Replace the grep/ack run-with-timer hack with helm-run-after-exit

[bbatsov]

#3 on the list. helm-projectile-grep (and the now-deprecated helm-projectile-ack) deferred the actual search through (run-with-timer 0.01 nil ...) - a fixed 10ms guess, present since the initial commit. The reason: when the command is invoked as an action from a Helm session (the C-s grep action in the project/directory sources), a new Helm session must not start until the current one has torn down.

That's exactly what helm-run-after-exit is for - and the codebase already uses it (the C-c p f etags binding). This routes both commands through a small helm-projectile--run-grep-or-ack helper that:

• defers via helm-run-after-exit when helm-alive-p (invoked from a live session), and
• runs the search directly otherwise (standalone M-x / keybinding).

So no more arbitrary 10ms latency on the standalone commands, no racy timer, and the deferral is explicit and self-documenting. Two tests cover both dispatch branches (68 specs).

Note: the interactive "grep from the project list" flow can't be exercised in batch, but in the real action paths Helm has already exited by the time the command runs (helm-exit-and-execute-action / action dispatch), so the direct-call branch handles them, with helm-run-after-exit as the guard for any still-live case.

• The commits are consistent with our contribution guidelines
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog
• You've updated the readme (no user-facing change)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 46.60%. Comparing base (56f81f1) to head (c0ba857).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
helm-projectile.el	80.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   46.33%   46.60%   +0.27%     
==========================================
  Files           1        1              
  Lines         559      560       +1     
==========================================
+ Hits          259      261       +2     
+ Misses        300      299       -1     

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