Add plugin system for external subcommands

[adrianreber]

This PR introduces a plugin architecture that allows extending checkpointctl with additional subcommands. Plugins are discovered by scanning PATH for executables named checkpointctl-<name>, where <name> becomes the subcommand. This approach keeps the main binary lightweight by allowing plugins with larger dependencies to be distributed separately, and enables contributors to implement plugins in any programming language.

• Auto-discovery of plugins in PATH at startup
• Plugin description support via --plugin-description flag
• Built-in commands take precedence over plugins
• First plugin in PATH wins for duplicate names
• New plugin list subcommand to show available plugins
• Documentation including man page and README updates

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.00000% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.51%. Comparing base (d4ddee6) to head (049219a).

Files with missing lines	Patch %	Lines
cmd/plugin.go	80.89%	10 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #202      +/-   ##
==========================================
+ Coverage   76.00%   76.51%   +0.50%     
==========================================
  Files          13       14       +1     
  Lines        1288     1388     +100     
==========================================
+ Hits          979     1062      +83     
- Misses        232      242      +10     
- Partials       77       84       +7     

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[github-actions]

Binary Size Check Failed

The binary size increase exceeds the allowed threshold.

Size Check Output

Changes from 06e23e6d1f24ce678daf67cfb85a3a1c2fba252d to 06e23e6d1f24ce678daf67cfb85a3a1c2fba252d:
Rebasing (2/6)
Executing: test/check-size.sh
go build -o checkpointctl -ldflags "-X main.name=checkpointctl -X main.version=1.5.0"
BINARY SIZE (13a7b8e): 9616680
No previous size present, storing current size
Rebasing (3/6)
Rebasing (4/6)
Executing: test/check-size.sh
go build -o checkpointctl -ldflags "-X main.name=checkpointctl -X main.version=1.5.0"
BINARY SIZE (fc59aac): 9672072
FAIL: size difference of 55392 B exceeds limit 51200 B
warning: execution failed: test/check-size.sh
You can fix the problem, and then run

  git rebase --continue

How to resolve

If this size increase is expected and acceptable, a maintainer can add
the bloat-ok label to this PR to bypass the size check.

---

Binary size check performed by CI

[github-actions]

Test Results

69 tests  +8   69 ✅ +8   2s ⏱️ ±0s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 049219a. ± Comparison against base commit d4ddee6.

[rst0git]

@adrianreber If I understand correctly, this is somewhat similar to kubectl's plugin mechanism. Is that correct?

[adrianreber]

@adrianreber If I understand correctly, this is somewhat similar to kubectl's plugin mechanism. Is that correct?

Exactly, that way the size of the binary does not increase automatically.