docs(examples): add runnable examples for helper packages

[ernado]

Part of #166. Adds runnable example binaries under examples/ for helper packages and for the concrete scenarios requested in the issue's comment thread. Each follows the existing example style (examples.Run/ClientFromEnvironment + Terminal auth) and is listed in examples/README.md.

Helper-package examples

Binary	Helpers demonstrated	Connects?
get-participants	peers.Manager resolve + query/channels/participants iterator (channels.getParticipants)	yes
bot-inline	message/inline results builder + message/styling, via OnBotInlineQuery	yes
takeout	takeout session helper (account.initTakeoutSession) + wrapped API calls	yes
deeplink	deeplink.Parse / IsDeeplinkLike + type routing — offline CLI, no login	no
dialogs	query.GetDialogs + query.Messages(...).GetHistory iterators	yes
rich-message	message/rich page blocks + inline rich text → Builder.RichMessage	yes
upload-url	uploader/source HTTPSource + uploader.FromSource streaming upload	yes

Examples requested in #166 comments

Binary	Request(s) addressed
send-message	"user login by code with session storage in file", "reconnect with the previous session", "send a message to a channel" (requested 3×)
save-media	"reply to a picture/video with key phrase 'save' and it'll save the referred media"
get-participants	"Please add example of ChannelsGetParticipants method usage"

Comment requests intentionally not added

• Add contact by phone number — the thread itself advises against it (account-ban risk); left out deliberately.
• Get a peer's phone number inside OnNewMessage — phone numbers are generally not exposed by the API due to privacy, so there's no meaningful example.

Notes

• examples/go.mod / go.sum gain one entry: the rich package transitively pulls in github.com/yuin/goldmark, added via go mod tidy in the examples module.
• All examples build, go vet clean, gofmt clean.
• Existing helpers already covered by current binaries (uploader→bot-upload, downloader→gif-download, etc.) were not duplicated.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.06%. Comparing base (5bd59ff) to head (ba538b4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1773      +/-   ##
==========================================
- Coverage   71.10%   71.06%   -0.05%     
==========================================
  Files         501      501              
  Lines       23501    23502       +1     
==========================================
- Hits        16711    16702       -9     
- Misses       5569     5574       +5     
- Partials     1221     1226       +5     

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