refactor: simplify workflow file loading

[nikpivkin]

Simplified the workflow file loading logic.

• Replaced filepath.Walk with filepath.WalkDir since it was introduced in Go 1.16 and is more efficient, also returning fs.DirEntry without extra fs.FileInfo conversion.
• Renamed WorkflowFiles to WorkflowFile because the structure represents a single file rather than a collection.
• File traversal is now uniform — filepath.WalkDir is used regardless of whether the path is a file or a directory.
• Recursive traversal logic is handled inside the walk function instead of using separate processing via os.ReadDir.

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.76%. Comparing base (bd4bc99) to head (0bd2949).
⚠️ Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/model/planner.go	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6017      +/-   ##
==========================================
+ Coverage   74.65%   76.76%   +2.11%     
==========================================
  Files          73       74       +1     
  Lines       11139     9203    -1936     
==========================================
- Hits         8316     7065    -1251     
+ Misses       2186     1503     -683     
+ Partials      637      635       -2     

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[nikpivkin]

It looks like the failed test is not related to changes in the code:

  - Using [grype v0.87.0] https://megalinter.io/8.4.2/descriptors/repository_grype
  - MegaLinter key: [REPOSITORY_GRYPE]
  - Rules config: [.grype.yaml]
  - Command: [grype --config /action/lib/.automation/.grype.yaml dir:.]
  --Error detail:
  [0000]  WARN no explicit name and version provided for directory source, deriving artifact ID from the given path (which is not ideal)
  NAME                           INSTALLED             FIXED-IN  TYPE       VULNERABILITY        SEVERITY 
  @octokit/plugin-paginate-rest  2.21.3                9.2.2     npm        GHSA-h5c3-5r3r-rr8q  Medium    
  @octokit/request               5.6.3                 8.4.1     npm        GHSA-rmvr-2pp2-xj38  Medium    
  @octokit/request-error         2.1.0                 5.1.1     npm        GHSA-xx4v-prfh-6cgc  Medium    
  github.com/docker/cli          v28.4.0+incompatible  29.2.0    go-module  GHSA-p436-gjf2-799p  High
  [0014] ERROR discovered vulnerabilities at or above the severity threshold